DOHA (Reuters) - Washington's hopes that U.S.-led forces would be welcomed into Iraq as liberators bled into the sand on Sunday, the fourth day of war, as Iraqi troops fought back with determination and guerrilla tactics.

There was no evidence of weapons of mass destruction being used by Iraq in battle. Instead, Iraqi troops were fighting with machinegun-mounted Japanese pickup trucks against squadrons of the world's most formidable battle tank, the U.S. Abrams.

There were reports of between 10 and 15 U.S. troops killed in fighting to secure bridgeheads across the River Euphrates at Nassiriyah, with perhaps up to 50 more wounded.

U.S. General John Abizaid acknowledged it was the "toughest day of resistance" so far. He said Iraqi forces near Nassiriya inflicted several casualties in "the sharpest engagement of the war." There were 12 American troops missing, he added.

"Everybody was predicting they'd be welcomed as liberators but it's working out differently," said one senior Arab official in the Gulf. "The Americans had a hard day today."

Evoking Vietnam and Mogadishu, Iraqi Information Minister Mohammed Saeed al-Sahaf warned U.S. forces they were driving into "a quagmire from which they can never emerge, except dead."

Iraqi forces evidently switched from their disastrous static defense of the 1991 Gulf War to classic guerrilla tactics, using loyalist militias to bolster regular forces.

"There are a number of incidents occurring to the rear of the main combat forces," Abizaid said at the Central Command briefing, indicating guerrilla-style attacks. He said Iraqis had pretended to surrender, then ambushed U.S. forces.

FEARFUL CAPTIVES

Despite at least 2,000 Iraqi surrenders, the picture was of a far more spirited fight by Iraq's troops than some analysts had predicted, slowing the invading forces' sweep from Kuwait through southern Iraq toward Baghdad.

Iraqis operating in small pockets or hit-and-run raids held up the U.S.-led advance in at least four places and captured their first U.S. prisoners, whom they displayed on television.

In grim pictures, Iraq showed four bodies of what it said were U.S. soldiers and five captives taken near Nassiriya, who said they were from a U.S. Army logistics support unit.

Abizaid called the pictures "disgusting."

The capture suggested that Iraqi forces, perhaps in small raiding groups, attacked the exposed flank of a U.S. armored advance which has plunged some 200 km (120 miles) north into Iraq in just 72 hours, stretching its lines of support.

Reports and television images of battling Iraqi units in the south came from reporters traveling with those American and British units. There was no hard information on the progress of other units who were not accompanied by journalists.

A U.S. military spokesman told the Central Command briefing there were movements deep in to Iraq "that we're not showing." Iraqis also paid with their lives for their attack on the U.S. tanks near Najaf, leaving bodies strewn across the desert. But Defense Secretary Donald Rumsfeld had to acknowledge that U.S. soldiers were dead and others captured and Britain said a Tornado ground attack plane with a crew of two had been downed by mistake by a U.S. Patriot missile.

Britain has already lost 16 men in non-combat incidents, with two helicopter crashes and the downing of the Tornado.

British Harrier ground-attack jets were brought in to pound an Iraqi redoubt near the Gulf port of Umm Qasr only after several hours of fighting, shown live on television, in which U.S. tanks apparently failed to break Iraqi resistance.

After night fell, some Iraqis were still holding out.

There was continuing Iraqi resistance at Basra, Iraq's second city in the far south, and, at Najaf, U.S. officers expressed amazement at pickup truck attacks, a tactic dating from the 1980's Chad civil war in the Sahara desert.

The Iraqis, while massively outgunned, were also using rocket propelled grenades, machine guns and small arms to good effect to pin down U.S. forces reluctant to risk casualties.

If the tactic worked well at Umm Qasr in the relatively open territory of a port-side industrial zone, its effectiveness could be multiplied on the outskirts of Baghdad where U.S. concern to avoid civilian casualties would be far greater.

Iraq's toughest troops are arrayed south of the capital.

In Kuwait, former oil minister Ali al-Baghli said the time taken to capture Umm Qasr might undermine any faith ordinary Iraqis had that the Americans could topple Saddam Hussein.

"We are astonished that there is still resistance in Umm Qasr after all this time. It is a very small place.

"If it takes them this long to capture Umm Qasr, how long will it take to capture Tikrit or Baghdad?"