An unspecified number of American troops were taken as prisoners even as US-led forces on Sunday advanced to within 150km of Baghdad facing pockets of resistance in various parts of Iraq.

US President George Bush told reporters at the White House that this was 'the beginning of a tough fight'.

The Pentagon in Washington was quoted by CNN as saying that some of the 10 US soldiers reported missing in southern Iraq may have been taken prisoners.

The confirmation of the troops being taken captive came hours after Qatar-based Al Jazeera news channel aired  footage of interviews with what the station identified as captured American soldiers.

The channel also showed bodies in uniform in an Iraqi morgue that it said were of Americans.

President Bush hoped that captured Americans would be treated 'humanely, just like we are treating prisoners we have captured'.

On Sunday, Baghdad came under fresh US-led bombardment on the fourth day of air strikes when a missile landed on an unidentified target on the east bank of the Tigris river, sparking a huge fireball.

In Baghdad, a series of air raid sirens and explosions were heard on the outskirts of the city at midmorning.

Iraqi officials said more than 500 Iraqis in four cities were injured in allied air strikes on Saturday and claimed that 77 civilians were killed in Basra, the main city in southern Iraq.

President Bush has said that a 'massive amount of humanitarian aid would begin moving into Iraq through the southern part, which coalition forces are trying to secure. The aid will start moving in once the area is completely safe'.

Six marines were reported killed at the strategic town of Nasairayah where Iraqi forces and US soldiers were engaged in a fierce encounter.

The US-led forces lost a British Royal Air force fighter, which was engaged by a missile battery that did not belong to the US or Kuwait or any other country. The fate of the British crew was not known.

The Al Jazeera channel also reported that Iraqi troops on Sunday found a two-member US or British air crew hiding in the reeds on the edge of the Tigris river in Baghdad. Iraqi officials claimed to have shot down their plane. The channel showed Iraqi soldiers searching for the crew firing into the river.

Meanwhile in Washington, US Defence Secretary Donald Rumsfeld claimed that his country's forces were making 'excellent progress' on the fourth day of the war and that thousands of Iraqi troops have surrendered.

"There are pockets of resistance and how long the war will last and how many casualties the US and its allies will suffer, no one can say," he said.

PTI