U.S. invasion troops have advanced quickly to the doorstep of Baghdad, leaving their commanders with a tantalizing choice: Continue the charge into Saddam Hussein s capital or wait for reinforcements and give Iraqis a chance to overthrow the regime themselves.But part of the price for the rapid advance was Iraq's first two successes in shooting down U.S. aircraft. In fighting near Karbala, 50 miles south of Baghdad, Iraqis on Wednesday downed an Army Black Hawk helicopter.

The crash of a Navy F/A-18C jet was still being investigated Thursday, though it also appeared likely it was downed by enemy fire, officials said.

The six Americans aboard the helicopter were killed, they said.

Lead units of the multi-pronged U.S. invasion Army and Marine assault force were about four miles from the edge of Baghdad, and some soldiers made a brief foray into a presidential palace near Saddam International Airport.

Special forces infiltrated some Iraqi command posts in the Baghdad area during the night, seeking strategic information, and also secured some bridges and dams to forestall possible sabotage, according to the U.S. Central Command.

The advance set the stage for either a final push on the capital or the capitulation of Saddam's best and most loyal fighters.

Some Pentagon officials said Wednesday the American forces likely would pause on the outskirts of the capital to allow pressure to build on the Iraqi regime, perhaps enough so it would fall without the chaotic and bloody urban fighting Iraqi officials say they are planning.

A pause also would allow more reinforcements to enter Iraq. The 4th Infantry Division, which has some of the Army's most advanced tanks and equipment, is arriving in Kuwait and could field a brigade-sized task force of a few thousand soldiers as early as Monday or Tuesday, the official said.

But top civilian leaders said they weren't counting on Saddam's capitulation. "It doesn't seem likely," said Victoria Clarke, spokeswoman for Defense Secretary Donald H. Rumsfeld.

Still, the Pentagon sought to lower expectations that the Iraqi capital could be taken quickly or easily.

"We are planning for a very difficult fight ahead in Baghdad," Maj. Gen. Stanley McChrystal told a Pentagon news conference. "We are not expecting to drive into Baghdad suddenly and seize it."

If American forces do battle to take Baghdad, whether after a pause or not, the strategy is likely to include cordoning off the city and targeting key sites for attack, a military official said.

The Americans would try to avoid the street-by-street battles that Iraq wants to set as traps by focusing on such key areas rather than trying to take over the entire city, the official said.

The United States lost its first confirmed aircraft shot down Wednesday amid the rapid advance. Pentagon officials said it was downed by small-arms fire.

There was no word early Thursday on the fate of the pilot of the Navy plane. Search and rescue operations were still underway Thursday for the jet, which was flying from the aircraft carrier USS Kitty Hawk.

U.S. military officials have weathered criticism over the past week as commanders, other Pentagon officials and outside analysts said the war effort had been slowed by unexpectedly stiff resistance and a plan that may have relied on too few troops.

McChrystal and Clarke told reporters at least six times in a 30-minute briefing that the toughest fighting may lie ahead.

McChrystal said a major American offensive had pushed closer to Baghdad amid resistance from Republican Guard troops that was "sporadic, but not able to stop coalition maneuvers."

It also was unclear whether those Iraqi forces had some of the chemical and biological weapons that U.S. leaders say Saddam is hiding - and whether they could or would use them.

From the start of the war, the Pentagon has hoped that under the threat of advancing forces the Iraqi military would surrender in great numbers and civilians might revolt. Officials also have warned from the start that the closer U.S. invasion troops get to Baghdad, the more likely Iraqi forces were to use chemical weapons.

The last line of defense for Baghdad may be Saddam's Special Republican Guard and Special Security Organization. It was not clear what damage had been done to these units inside the city.

PHOTO CAPTION

U.S. invasion Army tanks with the 3rd Infantry Division Task Force 1-64 move north near Karbala in central Iraq Wednesday, April 2, 2003. The Army's 3rd Infantry Division and the First Marine Expeditionary Force launched a two-pronged attack toward Baghdad, and both reported breakthroughs as units entered the so-called ``red zone'' within range of the guns and missiles defending the capital. (AP Photo/John Moore)