Iraqi forces ambushed U.S. troops today at a key Euphrates River crossing on the march toward Baghdad, killing an estimated 16 Americans and capturing at least five in two clashes that resulted in the largest number of U.S. casualties of the four-day-old campaign to take down President Saddam Hussein's government.

Iraqi television broadcast graphic images from a morgue showing uniformed bodies that U.S. officials tentatively concluded were among those of seven American soldiers believed killed in one of the encounters. The images, retransmitted by television networks around the world, also showed brief interviews with five other U.S. soldiers, apparently members of an Army maintenance company taken prisoner in the same clash.

U.S. officials expressed belief that the killed and captured soldiers seen on the videotape were 12 members of a supply convoy who were missing after they made a wrong turn outside the town of Nasiriyah, 100 miles northwest of the Kuwaiti border, and were fired upon by Iraqi militiamen.

Inside the city, as many as nine U.S. Marines also were killed after an Iraqi surrender turned out to be a "ruse," Army Lt. Gen. John Abizaid said at a briefing. The Marines died after a small group of soldiers who had indicated they wanted to give themselves up instead fired a rocket-propelled grenade at the Marines' amphibious assault vehicle, witnesses told journalists traveling with the Marine unit. Abizaid called the clash "the sharpest engagement of the war so far."

The casualties -- along with the disturbing images of bodies and captured U.S. troops, and a surge in resistance by Iraqi defenders in the south -- suddenly undercut the impression of irresistible U.S. might that had accompanied the invasion of Iraq for its first three days and led U.S. leaders to reinforce warnings that a difficult conflict could lie ahead.

[Hussein delivered a rambling and defiant speech Monday on Iraqi television, vowing that "victory will be ours soon." Dressed in a military uniform, he read from a prepared text without the glasses he wore during a speech televised Thursday after the first U.S. airstrike.

[Hussein made reference to "the war that started today," according to a simultaneous translation on CNN, leaving it unclear whether the speech was being broadcast live or had been recorded earlier. Senior U.S. and foreign officials have suggested Hussein may have been wounded and perhaps incapacitated in the first strike (Details, Page A15).]

The battles in Nasiriyah occurred as U.S. forces started a full-fledged deployment in northern Iraq's Kurdish-controlled area and the Army's 3rd Infantry Division closed to within 100 miles of Baghdad, passing by the sacred city of Najaf and nearing the front lines of elite troops arrayed to defend the capital. The pounding of Baghdad continued, meanwhile, as U.S. missiles and bombs hit intermittently with thunderous blasts aimed at government buildings and military installations.

[Early Monday, several squadrons of the U.S. Army's premier combat helicopter, the AH-64 Apache Longbow, attacked Republican Guard positions in Baghdad's southern suburbs. The attack force, which drew heavy fire from the Iraqi forces, destroyed four or five armored vehicles and several light vehicles, said Col. Bill Wolf, commander of the 11th Aviation Regiment, which carried out the strike.

["It was pretty fierce out there," said Wolf, who said complete damage assessment was still being carried out. One Apache returned to its base in west central Iraq with visibly heavy damage, and another crashed on takeoff but its crew was uninjured.]

But in the south, where the invasion began, U.S. and British forces struggled to wrest control of the port city of Basra and to pacify other areas they had previously seized and left in their wake, finding greater staying power than anticipated in Iraqi regular army troops and irregular security forces.

Fierce fighting was reported in Umm Qasr, a port on the Kuwaiti border where Iraqi defenders fought street clashes with Marines. A tank and artillery battle raged on the outskirts of Basra, a city of more than 1 million people on the Shatt al Arab waterway, at the head of the Persian Gulf. Iraqi officials said U.S. and British airstrikes on the city killed 77 people and wounded 366.

The resistance in Umm Qasr should be an "example" to the invading force, said Mohammed Saeed Sahhaf, the Iraqi information minister. "The fighters, the resistance, our heroes in Umm Qasr are teaching them a lesson."

In the first known friendly fire incident of the war, a U.S. Patriot missile battery shot down a British Royal Air Force Tornado GR4 warplane, killing two crew members, U.S. and British military spokesmen announced. The shoot-down occurred just outside Camp New Jersey in northern Kuwait around 2 a.m., when witnesses heard the roar of the launch followed shortly by another explosion.

British officials said they had not determined whether American error was responsible. They said the Tornado may have failed to emit an encrypted signal that would have informed U.S. forces that it was an allied aircraft.

[Early Monday, two British soldiers were reported missing when their vehicle was attacked in southern Iraq, according to news services. Officials with the British Defense Ministry declined to give details of which unit the soldiers belonged to and would not elaborate on where the attack happened.]

The attacks on U.S. forces in the Nasiriyah area and, in particular, the continued resistance in Basra and other parts of southern Iraq suggested that Iraqi troops and paramilitary units were mounting a stronger defense than anticipated, U.S. officials conceded.

"Clearly they are not a beaten force," Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, said on ABC's "This Week" program. "And those who think this is going to go on for some time are right. The hardest part is yet to come."

U.S. commanders had expected Hussein to adopt a scorched-earth strategy in response to an American invasion, perhaps using chemical weapons and sabotaging the country's vast southern oil fields. Some U.S. officials and analysts also predicted that large numbers of Iraqi soldiers would quickly surrender, that civilians would welcome U.S. troops as liberators and that a display of overwhelming force would trigger anti-government revolts, particularly in southern Iraq, which is dominated by Shiite Muslims who have little love for Hussein's Sunni-dominated government.

Those assumptions shaped American war plans. Instead of making a slow, deliberate push into the country, U.S. commanders focused on securing key installations and moving as quickly as possible toward Baghdad and other strategic targets, leaving difficult situations in the south for later resolution.

But four days into the war, fighters loyal to Hussein have not given up or faded away as thoroughly as commanders had hoped. Instead, some are doing what Iraqi officials have long promised: mounting guerrilla attacks and pulling back into cities in an effort to use civilians as shields and draw coalition forces into urban combat.

The Iraqi strategy has also resulted in U.S. troops facing threats not just in front but also in the middle and rear of their armored columns moving toward Baghdad. Nasiriyah, bypassed on Saturday by advance elements of the 3rd Infantry Division, was a bloody example.

Sahhaf, the information minister, said the United States and Britain would face a "quagmire" if they continued their advance on Baghdad. "We have drawn them into the swamp and they will never get out of it," he said.

Abizaid, deputy commander of U.S. forces in the Persian Gulf region, told reporters at Central Command regional headquarters in Doha, Qatar, that today was "the toughest day of resistance that we've had thus far." But he said it was "also a day in which we've continued the attack in almost every area. We understand that there may be other tough days ahead of us, but the outcome is still certain."

He said U.S. forces remained "on track" and would "arrive in the vicinity of Baghdad soon."

According to Abizaid, the United States has taken about 2,000 Iraqi soldiers prisoner -- far fewer than the tens of thousands who turned themselves in to U.S.-led forces during the Persian Gulf War advance into occupied Kuwait.

"The main reason that there haven't been a lot of mass surrenders on the same scale as in 1991 is that the Iraqi forces were really trapped in Kuwait. They were far away from home. They had nowhere to melt back to," Abizaid said. "Here, in the areas that we've been encountering regular Iraqi forces, by far the majority of units have just melted away."

In the most dramatic ground advance, the 2nd Brigade of the 3rd Infantry Division traversed about 230 miles in 40 hours, racing day and night across the desert to take up positions roughly 100 miles from Baghdad. At one point, the soldiers ran into 100 Iraqi militiamen who had pickup trucks armed with machine guns. The unit killed almost all of the Iraqis, according to journalists traveling with the soldiers.

The dead U.S. soldiers and five prisoners of war shown on Iraqi television were reported to be members of the Army's 507th Maintenance Company, based at Fort Bliss, Tex. The unit had been driving toward Baghdad to support the 3rd Infantry's rapid advance. The soldiers were traveling down a route that had been secured, but mistakenly took a wrong turn into an area with no U.S. combat forces.

"It was probably like many other tragic incidents in war, when a young officer leading his convoy made a wrong turn and went somewhere where he wasn't supposed to," Abizaid said.

The gruesome Iraqi television footage provided lingering, close-up images of the dead soldiers lying on the floor. Two of them appeared to have been shot in the head, one directly between his eyes and one in the forehead. An announcer for al-Jazeera, the pan-Arab satellite network that diffused the footage, said the video of the men's bodies was made in a morgue in Nasiriyah.

The footage also included brief and apparently spontaneous interviews with four men and a woman who identified themselves as American soldiers. All the prisoners appeared nervous and two appeared to be injured. A man was pictured lying on a cot, groaning and grimacing; his arms were bandaged and his face was bloody. A woman was shown sitting on a sofa, clutching her arms over her chest; she was barefoot and her left ankle was bandaged.

The Marines' Task Force Tarawa pulled into Nasiriyah today to take control from the Army as 3rd Division units moved north. Marine units set out quickly to secure two bridges traversing the Euphrates on the eastern side of town. But they were met by Iraqi army units armed with tanks, artillery and mortars on a two-mile stretch of road between the bridges. Separate paramilitary squadrons dressed in black also sniped at the U.S. troops with assault rifles and rocket-propelled grenades, U.S. officers reported.

In addition to the Marines killed in the grenade attack on their vehicle, about 50 others were reported injured in the fighting, U.S. military officials said.

Many of the casualties occurred after the Marines approached Iraqis who appeared to be surrendering but instead opened fire, Abizaid said. In one instance, Iraqi soldiers dressed in civilian clothes seemed to welcome U.S. troops and then shot at them. In another, Abizaid said, Iraqis raised a flag of surrender, then opened fire with artillery.

The six-hour battle ended only after the Marines called in air support from F/A-18 Hornets, AV-8 Harriers, A-10 Thunderbolt tank killers and AH-1 Cobra attack helicopters. The Marines reported destroying 10 Soviet-era T-55 tanks as well as an artillery battery and an antiaircraft gun. Marine officers made no estimate of Iraqi casualties.

With the 3rd Infantry's speedy march toward Baghdad, the ground war was building to what senior U.S. officials expect will be a climactic battle with Republican Guard forces, perhaps in the Karbala region about 50 miles north of Najaf.

Defense Secretary Donald H. Rumsfeld said in Washington that some Republican Guard forces have continued to follow a pattern evident over the last few weeks of moving toward Baghdad and Hussein's home town, Tikrit. But Myers said the force's two main divisions west of Baghdad -- the Nebuchadnezzar and the Hammurabi -- are still essentially in their same positions and U.S. warplanes are trying to contain them.

"Since the first day of the air campaign, we have been working on the Republican Guard divisions to prohibit them from, the best we can, from withdrawing back in closer to Baghdad," Myers said.

Air Force F-16s, B-52s and other aircraft have in the past two days battered Republican Guard units. Weather permitting -- a storm system is forecast this week -- AH-64 Apache attack helicopters from the 101st Airborne Division will concentrate on Iraqi armor and mechanized units shielding Baghdad.

Correspondents William Branigin, Peter Baker and Mary Beth Sheridan with U.S. forces, correspondents Alan Sipress in Doha, Qatar, and Karl Vick in Sulaymaniyah, Iraq, and staff writers Bradley Graham and Jonathan Weisman in Washington contributed to this report.

A Marine kicks open a door during a search outside Nasiriyah, a city on the Euphrates River where deadly clashes took place. Iraqi soldiers surrender to troops with the 3rd Infantry Division in the desert after a battle around Najaf. The division was attempting to bypass the sacred city on their way to engage the front lines of elite troops defending Baghdad.A Marine takes cover during searches of houses on the outskirts of the southern Iraqi city of Nasiriyah.