WASHINGTON -- A column of U.S. tanks rolled into the heart of downtown Baghdad early today and American troops seized a sprawling presidential palace compound along the Tigris River, the Information Ministry and a major hotel, according to reports from the scene.

Loud explosions were heard and a huge cloud of black smoke rose over the palace as a small number of dark-clad Iraqi soldiers were seen on television running along the west bank of the river. Some Iraqis were reported to have jumped into the river, attempting to flee.

The incursion signaled that U.S. forces intended to take control of the center of the capital in what could amount to the final drive to topple the regime of President Saddam Hussein.

The 2nd Brigade of the 3rd Infantry Division entered the city at 6 a.m., moving up Highway 8 and meeting only moderate resistance, mostly from infantry firing assault weapons and rocket-propelled grenades, the Associated Press reported.

Col. David Perkins told his troops before the operation that the mission was intended to be "a dramatic show of force" to demonstrate that U.S. forces can enter Baghdad at any time, anywhere, according to the AP.

The incursion today was the most daring U.S. military move since Saturday, when a long column of tanks made a three-hour strike deep into the city

Yesterday, despite pockets of Iraqi resistance, U.S. forces claimed control of all major roads into and out of Baghdad, and demonstrated sufficient command of the capital to land a large military cargo plane at the city's international airport.

A day after a violent three-hour incursion into Baghdad by scores of American armored vehicles that U.S. officials said left about 2,000 Iraqi dead, a senior Pentagon official cautioned that while Iraq's Republican Guard has been badly damaged, there is "still fight left" and potentially difficult combat ahead.

Rather than plunge into the city of 5 million, U.S.-led forces were attempting to encircle Baghdad. Marine Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, indicated that troops would stage rapid raids to destroy Baghdad's defenses while maintaining around-the-clock bombardment of Iraqi forces that emerge to fight.

Early today, the Associated Press reported that U.S. troops seized key buildings in the center of the Iraqi capital, including a major presidential palace and the Information Ministry.

Reporters saw tanks roll into the heart of Baghdad on the western side of the Tigris River, which divides the city. Also occupied was the Al-Rashid Hotel.

In the southern city of Nasiriyah, U.S.-led forces airlifted soldiers of an Iraqi exile group, which Pace called the nucleus of a future Iraqi army, to serve as humanitarian liaison officers and help root out Iraqi leader Saddam Hussein's paramilitary fighters among the population.

There were no major allied incursions into Baghdad yesterday, though probes were reported amid a day of progress and one notable setback.

The specter of Iraqi possession of chemical weapons hovered over yesterday's actions. An account by a Knight-Ridder reporter accompanying the 101st Airborne Division said dozens of U.S. soldiers were evacuated last night from an Iraqi military compound after a mobile laboratory confirmed the existence of the nerve agent sarin.

However, a spokesman for the U.S. Central Command in Qatar, Lt. Cmdr. Charles Owens, said that after checking, he could not confirm the report.

"We're aware of the Knight-Ridder story. We have no reports of that -- nothing to confirm it," Owens said.

U.S. officials have said repeatedly that they are certain that Iraq possesses chemical and biological weapons, but they have not been used in the conflict.

In the north, American officials acknowledged that U.S. aircraft were probably to blame for the mistaken "friendly fire" bombing of a convoy carrying U.S. Special Forces and Kurdish fighters. Kurds reported 18 of their soldiers dead and more than 45 wounded, including the son and brother of Massoud Barzani, one of the two top Kurdish leaders. One U.S. soldier was reportedly injured.

In the south, British troops made their largest incursion yet into Basra, Iraq's second-largest city. Three British solders were killed. The fall of the city appeared imminent.