Forward elements of the U.S. invasion force pushed to within 50 miles of Baghdad today, heading toward a potentially decisive battle with Iraq's Republican Guard despite hit-and-run attacks along the way. U.S. planes bombed heavily to weaken Iraqi defenses, but a formation of advanced Army helicopter gunships that joined the attack was forced to turn back after running into a hail of small-arms fire.

U.S. artillery joined the warplanes in raining down explosives throughout the day to soften up positions around Karbala manned by President Saddam Hussein's best-trained and most loyal soldiers -- Republican Guard divisions assigned to block approaches to the Iraqi capital. Troops and armor from the Army's 3rd Infantry Division and the 1st Marine Expeditionary Force moved toward the Guard units on both sides of the Euphrates River, getting in position for an assault that could open a path to Baghdad, the main target in the five-day-old U.S.-British invasion.

Iraqi soldiers and militiamen responded to the advancing U.S. columns mostly with guerrilla tactics, hiding in residential areas and firing antiaircraft guns, rocket-propelled grenades and AK-47 assault rifles at the U.S. military's most advanced battle helicopters. One of the Army's AH-64D Apache Longbows went down -- Iraqi authorities said it was shot down by a farmer -- and a number of others abandoned their targets.

The two-man crew from the Apache that went down was captured and displayed on Iraqi television in cream-colored jumpsuits, without apparent injuries. The Pentagon identified the two as Chief Warrant Officers Ronald D. Young, 26, and David S. Williams, 30, based at Fort Hood, Tex.

[Fierce fighting was reported Tuesday morning in the city of Nasiriyah, where U.S. Marines took heavy fire but succeeded in moving a convoy across two strategic bridges. The Reuters news agency reported the U.S. Marines came under attack as they attempted to pass through the city, which lies between two bridges -- one over the Euphrates river and one over the Saddam Canal -- that lie on the path to Baghdad.

[The Marines previously had been unable to control the route and took casualties on Sunday when Iraqi forces mounted a guerrilla counterattack. Overnight, U.S. forces lined the route with tanks and other armoured vehicles and began pushing a huge convoy of troops and vehicles through the city, according to a Reuters report from the city.

[At first light Tuesday, the convoy was fired on and responded with artillery and helicopter gunships. Shortly before noon, Reuters reported, the convoy had passed over both bridges and resumed its advance toward Baghdad.

[Meanwhile, a heavy sandstorm predicted to last up to two days blew into Iraq overnight, presenting a potential complication for military operations around the country. The storm reduced visibility to 500 yards in areas just south of Baghdad, the Reuters news agency reported Tuesday morning, and U.S. officials said they expected conditions to worsen.]

Although ground forces continued their march north beyond Nasiriyah, unconventional and unabated resistance hindered U.S. and British military activity across a wide swath of southern Iraq.

Iraqi soldiers and militiamen, basing their Soviet-era tanks and artillery in residential neighborhoods to discourage attack, held off British forces at the southern port of Basra, Iraq's second-largest city with more than 1 million inhabitants. Diehard guerrilla forces also persisted in sniping at British troops in Umm Qasr, the country's main port at the head of the Persian Gulf.

Elsewhere in the south, which U.S. forces have largely left behind in their advance, Iraqi troops armed with rocket-propelled grenades tried to ambush British outposts and U.S. supply convoys by laying land mines on roads, setting booby traps and sniping from behind sand dunes, British officers said.

Hussein and his military strategists appeared emboldened by the capture of the Apache this morning and two clashes on Sunday during which at least 16 Americans were killed and five were captured. Iraq's state-run television broadcast a speech by Hussein in which he assured Iraqis that "victory will be ours soon."

Despite another series of punishing U.S. bombing attacks on Baghdad, a relaxed-looking Hussein mentioned the resistance of Iraqi forces in the south in an apparent attempt to dispel suspicions that he was killed or injured in a cruise missile attack as the war began Thursday.

In another sign that may reflect confidence, U.S. officers have received preliminary reports indicating Hussein has ordered some Republican Guard troops out of Baghdad and toward the main lines of defense to the south and east of the city. Their mission, the officers said, would be to reinforce the elite divisions charged with fending off the American invaders around Karbala, about 50 miles southwest of Baghdad, between Razzaza Lake and the Euphrates. With U.S. troops closing in on those positions, British Prime Minister Tony Blair warned that allied forces would face "a crucial moment."

So far, the initial Republican Guard units confronted by U.S. forces have used trickery and traditional defensive measures to hold their ground, U.S. officers reported. Commanders with the 3rd Infantry Division said they suspected Iraqis in civilian clothes were acting as spotters for artillery units. As a result, they ordered searches of suspicious cars that seemed to be dogging U.S. convoys. Soldiers reportedly found uniforms and weapons in some of the vehicles they searched.

Army Gen. Tommy R. Franks, the overall commander of the invasion, said his troops' progress "has been rapid and in some cases dramatic" despite the continuing clashes in the south. At the same time, additional troops began moving toward the theater, apparently for duty during what is expected to be an extensive U.S. and British occupation.

A Pentagon official said soldiers from the Army's 4th Infantry Division, which had planned to enter Iraq from the north via Turkey, instead began flying to the Persian Gulf region from their base at Fort Hood. The division, whose equipment was being transported aboard 35 cargo ships now in the Red Sea after being refused permission to unload in Turkey, is expected to be in Iraq by mid-April, the official said.

After Sunday's fighting at Nasiriyah -- an Army maintenance convoy was ambushed and a Marine amphibious assault vehicle was hit by a rocket-propelled grenade fired by soldiers feigning a surrender -- some U.S. military officials conceded that they may have underestimated the resolve of Iraqi troops and paramilitary units and overestimated the greeting U.S. troops would receive from the population.

"There is some dogged determination on the part of the Iraqis to get in and get fighting," one defense official said. Another Pentagon official conceded that the military may have "oversold" to itself the impact of an information campaign urging Iraqi solders to surrender. To date, U.S. planes have dropped 28 million leaflets telling Iraqi troops to rise the white flag.

But Franks, in his second news conference of the war, expressed confidence that his plans were proceeding apace. Asked about Iraqi resistance, he said: "I actually have seen no surprise here, and I think our people on the ground have not seen a surprise."

He emphasized that Iraq's military command-and-control structure has been significantly weakened since the start of the hostilities, saying, "Many orders which have been given by this regime have not been obeyed by a great number of his [Hussein's] subordinates."

Franks also said airstrikes have been weakening Republican Guard positions that ring Baghdad, whose soldiers are considered to be more disciplined and better-equipped than regular army units. Particularly heavy bombing was reported against the Medina Division, north of Karbala, and the Baghdad Division, near Kut, 100 miles southeast of the capital.

"They will continue to be hit at points and places and times that make sense to us," he said. "The effect has been very positive for us."

Far to the northwest, the official Syrian news agency reported, a U.S. missile hit a passenger bus carrying Syrian civilians fleeing the conflict, killing five and wounding 10. The Pentagon described the incident, which occurred Sunday, as an accidental strike by a U.S. warplane bombing a bridge.

Among those participating in the air assault against Iraqi defenses were Marine warplanes, which escalated attacks on Republican Guard units near Baghdad as well as regular army units in southeastern Iraq. According to a Marine tally, the service's pilots made 380 flights providing close air support in the last 24 hours, destroying two tanks, nine other vehicles, a pair of antiaircraft batteries, two military buildings, a command post, four ammunition storage depots and three fuel storage facilities.

If Hussein orders some Republican Guard forces out of Baghdad to meet the advancing U.S. forces, Marine commanders said it could work to their advantage because the Iraqi defenders would be in largely open areas where they could be targeted by bombing. If the defenders stayed in an urban environment, it would be harder to attack them for fear of civilian casualties.

In the meantime, heavy fighting continued through much of southern Iraq. British forces fought to regain control of two strategic areas -- the Rumaila oil fields and the Faw peninsula in far southeastern Iraq -- that had been listed as captured by U.S. and British troops in the first two days of the war.

Several Iraqi T-55 tanks pushed into the northern end of the oil fields to begin the clash. Troops from the British 16th Air Assault Brigade attacked them with anti-armor rockets and armored vehicles. Not far away, in the northern end of the Faw peninsula, the Queen's Dragoon Guard reported encountering a battalion of armor or mechanized infantry and engaged it in battle; they then pulled back and called in air support.

More fighting also broke out around Umm Qasr, where U.S. and British forces hope to bring in humanitarian aid soon. British naval minesweepers have been trying to clear a channel to the port.

One British soldier was reported killed in fighting, the first British combat death in the war, while two British bomb disposal soldiers were missing after their vehicle was destroyed by enemy fire or ordnance, officers reported. The battle in the oil fields was so fierce that an escorted tour for journalists was canceled by military officials, and civilian firefighters who were trying to put out eight well fires fled the area.

Other British forces also met continued opposition on the outskirts of Basra, but held off trying to force their way in and hoped to negotiate entry. British officers declared Basra's airport "contained," but had not entered for fear of advancing within range of Iraqi artillery.

Basra's water supply was disrupted Friday after the fighting resulted in damage to the city's main pumping facility. The International Committee for the Red Cross, which has been unable to send repair crews to the plant because of the combat, warned that the situation was "close to what we call a humanitarian crisis."

U.N. Secretary General Kofi Annan called for "urgent measures" to restore the city's water supply.

Maj. Fraser Smith, a British liaison officer at the U.S. Marine headquarters, said the persistent low-level attacks in southeast Iraq felt familiar to British commanders. "It's coming down to . . . a Belfast situation," he said. "It's civil unrest," rather than organized military opposition.

Marines operating in Nasiriyah, about 180 miles southeast of Baghdad, found the bodies of two Army soldiers who were caught in the ambush Sunday. Several other soldiers who came under fire in that attack were either killed or captured by Iraqis, who later displayed bodies and five prisoners on television.

In addition to the ambush, the Marines found themselves in a six-hour gun battle there Sunday that ended only when fighters and attack helicopters were summoned to help. U.S. military officials said nine Marines were killed and at least 64 were wounded in the fighting.

The Marines' Task Force Tarawa worked to secure the area today while other Marines crossed the two bridges captured Sunday and headed toward Baghdad along the eastern side of the Euphrates to join the 3rd Infantry Division in pressuring the capital. The Marines left behind in Nasiriyah continued to come under attack.

"They're still getting sporadic small-arms fire," said Lt. Col. David Pere, senior watch officer at the Marine command center in Iraq. "It's desultory. It's harassing. . . . They pop around a building and start spraying them."

Franks acknowledged that his strategy of marching quickly toward Baghdad has meant troops have intentionally bypassed areas of Iraqi resistance in the south, where the Iraqis may continue to harass U.S. forces and their lengthening supply lines. He predicted that U.S. and British forces would be involved behind front lines for days in fighting with Iraqi paramilitary units -- among them Saddam's Fedayeen, a militia run by Hussein's elder son, Uday.

"We know that the Fedayeen has in fact put itself in a position to mill about, to create difficulties in rear areas, and I can assure you that contact with those forces is not unexpected," he said. "You can expect that our cleanup operations are going to be ongoing. It isn't that we don't know where they are."

Franks said intimidation by Iraqi security agents in cities such as Nasiriyah and Basra has prevented the civil uprisings that some Pentagon planners had expected. "It's fear, the practice of this regime over a long period of time," he said. "Fear tactics are still being applied in many of these locations."

Baker reported from Marine headquarters. Correspondents Alan Sipress in Doha, Qatar, Keith B. Richburg in southern Iraq, and William Branigin and Mary Beth Sheridan with U.S. forces, and staff writer Jonathan Weisman in Washington contributed to this report.

Marines from the 2nd Battalion, 8th Regiment run to take positions as their camp is attacked by Iraqi forces around the key southern city of Nasiriyah. Marines from the 2nd Battalion, 8th Regiment carry the body of an Army soldier who was among those killed Sunday in an ambush on a convoy in Nasiriyah. The photo was released yesterday. Soldiers with the 3rd Infantry Division, after disembarking from a Bradley Fighting Vehicle, surround an Iraqi man who was stopped for suspicious activity. An AK-47 assault rifle and ammunition were found in the man's vehicle.