WASHINGTON - Fast-moving American invasion forces bore down on Baghdad yesterday, as U.S. missiles thundered into Saddam Hussein's palaces and offices for a second straight day, turning still more of them into so many piles of rubble.

Across the vast Iraqi desert, U.S. convoys roared virtually unimpeded along the road to Baghdad, crossing the Euphrates River and moving to within 100 miles of the capital. American troops are moving at a rapid clip, military officials said, roughly four times faster than allied forces were able to maneuver in the 1991 Persian Gulf war, when they were slowed by opposition from Iraq's army.

When they needed it, ground forces summoned firepower from above. U.S. Cobra helicopters fired missiles at 20 Iraqi tanks defending bridges outside the port of Basra. The U.S.-led coalition seized the international airport, located northwest of Basra, but had not yet gained control of Iraq's second-largest city.

Speed was a priority; unprofitable engagement was avoided. The troops stayed away from a street fight when they could. As the coalition commander, U.S. Gen. Tommy Franks, put it yesterday, "This is about liberation, and not about occupation."

Though U.S. forces said earlier they had secured the nearby port town of Umm Qasr, another day of fighting was reported there. Gaining control of the town was considered necessary to allow humanitarian aid supplies to be shipped into Iraq.

Just across the border in Kuwait, one of the most damaging attacks on American forces thus far may have been self-inflicted. Fourteen members of the 101st Airborne Division were wounded by grenade fragments in an assault on a command tent at Camp Pennsylvania. One of the men was reported to have died.

A soldier from the 101st was detained by military police, and a spokesman for the unit said a criminal investigation had been launched into the incident.

Overall, U.S. casualties were described as light, though the number of Americans killed since the war began four days ago reached seven when two British helicopters collided over the Persian Gulf early in the day. A U.S. Navy lieutenant was among seven on board killed in the crash, which British authorities said was not the result of enemy fire.

Iraqi government officials said that seven people had died in the U.S. airstrikes on Baghdad in the past two days, as nearly every U.S. warplane in the region attacked the capital city. The International Committee of the Red Cross said that at least 100 people were injured in the bombing. The Arab TV station al-Jazeera reported that 50 people had been wounded in the fighting in Basra.

At least five people were killed in a car-bombing near the Iraq-Iran border, apparently in retaliation for a U.S. cruise missile strike Friday night against facilities of the Ansar al-Islam, which Americans say is a terrorist organization connected to al-Qaida.

A cameraman for Australian television was among the dead in that attack, and elsewhere in Iraq a three-man crew from Britain's ITN television network was missing and presumed dead. It was a dangerous day for those reporting the war: Other groups of reporters found themselves under fire, and three were reported hiding, awaiting rescue.

The car bombing occurred after about 40 cruise missiles hit targets that included what U.S. sources called a factory that produces the poison ricin.

In a possible preview of the coming battle for Baghdad, new elements of the U.S. war-fighting strategy began to emerge from the battles in Basra, near the Persian Gulf coast.

American commanders are attempting to cordon off the city, rather than occupy it, in an effort to avoid the deadly risks of urban combat. The goal is to gain control over population centers without actually sending U.S. soldiers into crowded areas where snipers would find them easy targets.

"Our intent is not to move through and create military confrontations in that city," General Franks said. "Rather, we expect that we'll work with Basra and the citizens in Basra."

Setting up bases

That strategy of sealing off cities and towns was evident, as well, in Nasiriya, about 90 miles northwest of Basra, which was cordoned off by American ground forces as they rolled past on their way to Baghdad. As U.S. troops move northward, and expand their control of Iraq, Army units are setting up mini-bases, known as lodgements, in the desert. Apache Longbow helicopters and other weapons are being moved from Kuwait into these temporary bases, where they can be used to attack targets throughout the country.

Franks, who briefed reporters in Qatar, said American troops are performing "magnificently" and that the fighting has gone according to plan.

But like every other senior U.S. official, from President Bush on down, who has spoken publicly about the war in recent days, Franks warned against overconfidence.