According to a wire report, artillery fire could be heard near Saddam International Airport, tracer rounds raced through the blackened sky and artillery shells exploded in the air.
Officers of the 3rd Infantry Division told an Associated Press reporter assigned to the unit that the attack on the airport had begun.
The entire city of 5 million people appeared to be without power Thursday night as loud explosions thundered through the area.
Gen. Richard Myers, chairman of the U.S. Joint Chiefs of Staff, said during a Pentagon briefing that U.S. forces had not targeted Baghdad's power grid and were looking into the causes of the outage.
Earlier, U.S. Army and Marine assault forces had reached the outskirts of Baghdad and some soldiers raided a presidential palace about 55 miles north of Baghdad.
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.
They may stop and do some probing, but I think it's very unlikely they'll wait for the 4th Infantry Division," says CBS News Analyst Gen. Perry Smith. "They've got the momentum, they might as well go forward.
"They might not go directly to the middle of the city. One of the key places they want to grab is the International Airport."
Once this is accomplished, Smith says, "We can start flying stuff in which could be very, very helpful."
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.
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 Victoria Clarke, spokeswoman for Defense Secretary Donald H. Rumsfeld, 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. 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.