Five US soldiers have been captured and paraded on Iraqi television as coalition forces met determined resistance in the push to Baghdad.
Iraqi resistance has been stronger than expected
In what commanders called the "sharpest engagement" of the war so far, US Marines battling Iraqi forces in and around the southern city of Nasiriya have reported at least six of their number dead and 14 wounded.
The US military also confirmed that a six-vehicle supply convoy was ambushed in the area by Iraqi troops, and that 12 of its personnel were missing.
We expect them to be treated humanely, just like we'll treat
any prisoners of theirs that we capture humanely
US President George W Bush
Iraqi television broadcast pictures of the bodies of several American soldiers and their wrecked vehicles, as well as interviews with five captured survivors of the ambush - one of them a woman.
The Iraqis claim to have killed a total of 25 Americans in the area, and to have destroyed 30 tanks and armoured vehicles across southern Iraq. Iraqi losses, both in Nasiriya and throughout the country, are unclear.
In Kuwait, the defence ministry said an Iraqi missile had been intercepted by a Patriot missile in the north of the emirate on Sunday. If confirmed, this would be the 13th Iraqi missile fired at Kuwait since the war began.
US troops and armour pushing north from the southern city of Basra have reached the holy town of Najaf, just 160 kilometres (100 miles) from Baghdad.
But other American units are still engaged in fierce fighting in Nasiriya, a key crossing point on the Euphrates river, and other areas of southern Iraq.
Coalition Central Command has accused Iraqi special units - some of President Saddam Hussein's most loyal soldiers - of disguising themselves as civilians, and of firing on coalition troops after initially signalling that they were surrendering.
US Army General John Abizaid said: "It's the toughest day of resistance that we've had thus far. We understand that there may be other tough days ahead of us, but the outcome is still certain."
And he insisted: "We are on track. We will arrive in the vicinity of Baghdad soon."
General Abizaid described the broadcast of footage of dead and captured American soldiers, carried round the world by the Arabic television news channel al-Jazeera, as "disgusting".
US Defence Secretary Donald Rumsfeld said earlier that television networks showing interviews with captured soldiers were breaking the Geneva Conventions governing prisoners-of-war.
The southern Iraqi port of Umm Qasr saw renewed fighting on Sunday.
The confrontation began when a US Marine patrol came across a large group of Iraqi soldiers - some of whom are thought to have been from the elite Iraqi Republican Guard - who had been hiding in a compound in the new port area.
The Iraqi Foreign Minister, Naji Sabri, told the BBC that American and US troops would be defeated. But he promised that prisoners would be treated well.
However, some correspondents says elements of the coalition advance are encountering little resistance, with many local people waving and giving thumbs-up signals to American troops.
Meanwhile, Baghdad has come under a fresh, intense aerial bombardment.
Correspondents said parts of the city were on fire and shrouded in smoke, after a series of heavy explosions. Iraqi forces responded with anti-aircraft fire.
The latest attacks followed bombing raids in the late afternoon and evening.
The BBC's John Simpson in northern Iraq says there has been heavy fighting to the south of the city of Irbil.
He says that, according to a local Kurdish commander, the battle was between Iraqi soldiers trying to defect and a loyal army unit.
US President George W Bush said on Sunday that he expected a massive humanitarian aid programme for Iraq to begin within the next two days.
His comments came after the International Red Cross warned of a humanitarian emergency in Basra.
A spokesman told the BBC that water and electricity supplies to the city had been cut off.
The Iraqi Government said 77 civilians had been killed in attacks on Basra by coalition planes, with many more wounded.
In other developments:
- The US accuses Russian companies of selling arms to Iraq in breach of United Nations sanctions
- Coalition military spokesmen deny that any of their planes are missing over Baghdad, despite rumours sweeping the city that two parachutists were seen coming down into the River Tigris
- The two crew members of a British RAF Tornado aircraft are killed after apparently being shot down by a US Patriot missile near the Kuwaiti border with Iraq
- British intelligence reports suggest Iraqi President Saddam Hussein emerged alive from the initial US air raid on Baghdad but left the area in an ambulance, Foreign Office Minister Mike O'Brien says
- British news organisation ITN says reporter Terry Lloyd, missing near Basra, is now thought to be dead
- Protests against the war in Iraq continue around the world for the fourth consecutive day.