Iraqis on Wednesday were asking who was running their country after US forces swept almost unchallenged through northeastern Baghdad towards the heart of the capital and cheering looters sacked shops and offices.

There was no sign of police or government authority on the streets of central Baghdad.

Information ministry officials who had been shadowing foreign reporters through nearly three weeks of war were nowhere to be seen at the Palestine Hotel on Wednesday.

"You are a journalist. Please tell me what is going on. Where is our government? To whom do we belong now? I don't know," said Ammar Moussa, a shopkeeper visiting his wounded son at a Baghdad hospital.

"I want to know when all this mess is going to finish. There is no radio, no television. Is our government still in control or not?" asked Sarmed Shakir, a cleaner at the hospital.

Baghdad radio, off the air since US tanks stormed into the western half of the city centre on Tuesday, could be heard transmitting very faintly on Wednesday. It was broadcasting only patriotic songs. Television remained blank and silent.

Witnesses said looters had taken over the United Nations headquarters in Baghdad.

With no sign of the Iraqi police, looters also robbed shops around the heavily bombed buildings housing the Iraqi Olympic Committee, headed by President Saddam Hussein's elder son Uday.

Travelling with a unit of the 1st Marine Expeditionary Force, a Reuters  correspondent saw the soldiers encounter very little opposition, getting a largely warm reception as they swept through the poor residential district of Saddam City, in the northeast of the Iraqi capital.

The district is home to around two million Iraqis, mainly from the Shi'ite Muslim majority, who have traditionally been marginalised by the Sunni ruling elite, most lately by Saddam's Sunni-dominated government.

Some local residents came out on the streets and cheered the US troops.

US planes flew low overhead and marines laid down a long barrage of US artillery, which appeared to be ‘prep' fire, aimed at clearing a space of Iraqi soldiers for advancing American troops.

On Tuesday, the marines had swept through other parts of east Baghdad, meeting only sporadic resistance from Iraqi fighters. Marine officers on Wednesday said they expected to eventually meet up with army forces moving from the west.

Across the capital, parts of the 1st Brigade of the US 3rd Infantry Division prepared to thrust into the centre of the city from their base at the international airport in the southwest.

The infantry's 3rd Brigade in the northwest was sweeping down to the west of the Tigris River, which divides the Iraqi capital, and armoured units of the 2nd Brigade, which hold a presidential palace compound in the heart of Baghdad, were slowly expanding their operations.

"I can see American troops driving around in Humvees in the compound. They are that confident," said Reuters correspondent Khaled Yacoub Oweis, speaking from the Palestine Hotel, which overlooks Saddam's presidential palace across the Tigris.

Streets around the Palestine Hotel, which is situated on the eastern bank of the Tigris, were empty.

"We can't hear anything. Not even cars," correspondent Khaled Oweis said. "Near to us we can hear occasional machinegun fire, but it's eerily quiet.”

"You can actually hear the birds singing. In peacetime, you never heard them singing."

But despite their success in seizing key buildings in the centre of Baghdad, US officials have stressed on the continued dangers facing their forces as they tighten their grip on the city of five million.

Iraqis fired a rocket-propelled grenade across the Tigris River towards US tanks on the western bank early on Wednesday, one Reuters reporter said, adding that it seemed to come from an area close to the Palestine Hotel.

On Tuesday, a US tank fired on the hotel, killing two cameramen from Reuters and a Spanish television channel.

(Sean Maguire in Baghdad contributed to this report)