US tanks and low-flying jets have pounded Iraqi forces in central Baghdad in support of troops advancing across the city.
US tanks rolled onto a key bridge on the Tigris
But the Americans have met fierce resistance in some areas and the Pentagon said it believed the Special Republican Guard still had "great potential for some sharp fights".
The Americans have been pushing into the city from the north and south-east, as well as trying to extend an area they control in the city centre.
During the day, US marines seized the Rashid military airbase, five kilometres (three miles) south-east of the centre.
And in the north-east, another group of marines entered the sprawling mainly Shia Muslim suburb of Saddam City. A BBC correspondent travelling with them says they were greeted by smiling and waving Iraqis.
Two US 3rd Infantry tanks also rolled onto the Republic (Jumhuriya) Bridge in central Baghdad to attack Iraqi fighters dug in on the eastern bank.
But the BBC's Andrew Gilligan says that as night fell, US troops remained in their enclave on the western bank.
The streets of Baghdad, he says, are now eerily deserted.
Other key developments:
- Three journalists - one from Arabic news channel al-Jazeera, one from Reuters news agency and one from a Spanish television station - were killed in central Baghdad, two of them after the Palestine Hotel was hit.
- US military officials assess the results of an air raid on a residential area of Baghdad designed to kill Saddam Hussein, which is believed to have left at least nine civilians dead - but UK security sources believe the Iraqi leader escaped it
- The International Committee of the Red Cross expresses concern about the situation in Baghdad's hospitals, where doctors say they are being overwhelmed by the number of casualties
- British soldiers in the southern city of Basra say they must make it more secure before they can deal with widespread looting.
- US President George W Bush and UK Prime Minister Tony Blair have said they are both committed to involving the United Nations in the post-war reconstruction of Iraq.
The 3rd Infantry force in central Baghdad is believed to be aiming to link up with the US marines fighting their way in from the south-east and more 3rd Infantry units coming in from the north-west.
US ground-attack planes and helicopters strafed Iraqi targets across Baghdad on Tuesday - one ferocious attack by an A-10 'Tankbuster' plane left the planning ministry building ablaze.
Our correspondent says about 500 Iraqi fighters were bussed into battle, and were also attacked from the air.
The Americans also say they are maintaining control of all major access roads around the city.
"We've essentially isolated the capital," said a Pentagon spokesman, Major-General Stanley McChrystal.
Hundreds of civilians are reported to be trying to flee the city.
One US A-10 plane crashed near the American base at the international airport south-west of Baghdad, apparently after being shot down. US Central Command said the pilot had been rescued.
Despite the evidence of American advances, Iraqi Information Minister Mohammed Saeed al-Sahhaf has appeared again to tell reporters that the American forces are being dealt with.
"They are going to surrender or be burned in their tanks," he said.
He brushed off a suggestion that it was time for the Iraqi Government to surrender.
Meanwhile, the US military has said that more tests are required to determine if substances found at two sites in Iraq are banned chemical agents.
The US and Britain launched the war to destroy Iraq's alleged weapons of mass destruction.