The U.S.-led drive toward Baghdad pressed forward today as heavy bombardment of the Iraqi capitol continued from the air, and top Pentagon officials insisted that their plans to force the government of President Saddam Hussein from power were on track.

Defense Secretary Donald H. Rumsfeld, speaking to reporters between television interviews, said Iraqi resistance "has been in pockets quite stiff. It's going to get more difficult as we move closer to Baghdad," where Hussein's most-loyal and battled-tested Republican Guards are waiting.

"I would suspect that the most dangerous and difficult days are still ahead of us," Rumsfeld said.

Rumsfeld would not speculate on how long the war would take to accomplish its objectives, saying, "It's going to end with the Iraqis liberated."

In Baghdad, U.S.-led forces stepped up their air attacks on Iraqi forces defending the city as U.S. forces continued to advance on Hussein's capital. Preceded by punishing airstrikes, Army troops made their northernmost advance, fighting running battles with Iraqi soldiers and militiamen along a key road. Forces from the 3rd Infantry Division ended the day within several miles of Karbala, taking control of a stretch of a highway about 14 miles west of the town of Hillah. That put the soldiers 20 miles southwest of the ruins of Babylon and about six miles from the Euphrates River.

East of Karbala, soldiers of the 82nd Airborne Division clashed with Iraqi military and militia groups for a second day near Samawah, a city about 150 miles south of Baghdad on Euphrates River.

In the southeast, British forces took 200 prisoners in the town of Abu al Khasib, east of Basra. Royal Marine Commandos first said an Iraqi general was among those captured, but British military spokesman Will MacKinlay later told BBC television that the report was wrong, attributing the mistake to "the fog of war." A British soldier was killed in action in the area.

Three U.S. Marines were killed when a UH-1 Huey helicopter taking off after refueling at a forward operating base in southern Iraq crashed. A fourth Marine was injured and hospitalized.

Meanwhile, U.S. and British forces in Iraq and around the Gulf region braced for unconventional attacks, such as the explosion of a taxi at a checkpoint in Iraq that killed four U.S. soldiers yesterday.

In Kuwait, a man in civilian clothes drove a pickup truck into a group of U.S. soldiers outside a base store at Camp Udairiat, injuring 15, U.S. military officials said. Today's attack in Kuwait, the staging ground for the bulk of U.S. and British forces that have entered Iraq, was not the first on Americans there since the buildup to war with Iraq began last year. In October, a shooting killed one Marine and wounded another, and in November another shooting wounded two U.S. soldiers. In January, a gun attack killed a U.S. civilian contractor and injured another American near the U.S. Central Command headquarters in Doha, Qatar.

A short time after Sunday's attack, Gen. Tommy R. Franks, commander of the U.S. Central Command, told reporters in Doha that he was not aware of any link between Saturday's suicide attack and today's attack on his soldiers in Kuwait. "It was obvious that the modus of the second attack was not at all like the first attack," Franks said.

But Franks said his forces were on guard, saying it was "not at all remarkable that a dying regime would undertake such tactics as suicide bombers. Remarkable though is the connection all the way to the top of the Iraqi regime. . . . That attack was just endorsed by those in power in Baghdad. Remarkable."

In an interview on NBC's "Meet the Press," Iraqi ambassador Mohammed Douri defended his country's tactics. "We have the right to defend ourselves," he said. "This is the right enshrined . . . in the charter of the United Nations. We are using whatever we have as weapons to defend ourselves." Douri said his country's arsenal does not include chemical weapons, as the United States and Britain have said.

Franks spent much of his hour-long press briefing today challenging news reports about disagreements among U.S. military commanders and administration officials about war planning.

Franks gave a largely upbeat assessment of the 11-day old conflict, noting that the coalition had secured oil fields in the south, gained control of the coastline and moved to within 60 miles of Baghdad. "We're in fact on plan. And where we stand today is not, in my view, only acceptable, but truly remarkable," Franks said.

Franks said U.S.-led forces had destroyed what he called a "huge terrorist facility" in northern Iraq, apparently referring to joint U.S.-Kurdish attacks on Ansar al-Islam, a radical Islamic group. The U.S. has suspected the group of having ties to Osama bin Laden's al Qaeda network.

Franks denied reports that he had asked the Pentagon to delay the attack after Turkey's government rejected the a U.S. request to allow ground forces to enter northern Iraq from there. "The plan you see is the plan you we've been on," Franks said.

Rumsfeld made similar points in broadcast interviews today, denying reports that he had rejected requests from U.S. war planners for additional troops. "The planners are in the Central Command. They come up with their proposals and I think you'll find that if you ask anyone who's been involved in the process from the central command that every single thing they've requested has in fact happened," Rumsfeld told "Fox News Sunday."

Appearing on ABC's "This Week," the defense secretary insisted that the operation was unfolding exactly as it was planned. "There are more troops on the ground, and they're coming in every day. This has been planned, that was what his plan was. I haven't signed a new deployment order in days."

Rumsfeld said the United States has "no plans for pauses or cease-fires" in its war against Iraq.

Washington Post correspondents Peter Baker, William Branigin, Monte Reel and Alan Sipress, the Associated Press and Reuters contributed to this report.