No occupation forces spokesman was prepared to comment on casualties after the Monday morning attack, but as many as 12 marines may be dead.
An Iraqi policeman, who refused to give his name, said he saw "three US soldiers wounded or killed in each vehicle".
Heavy black smoke could be seen rising above a chemical storehouse that appears to have been booby-trapped.
One witness, Imad Hashim, said the bomb went off "when they [US marines] tried to force their way in, there was a huge ball of fire and I was thrown to the ground".
Hashim said he was about 100m away from the blast.
Four Humvees were seen burning as US forces closed off the area, residents said they also saw rocket-propelled grenades being fired into the Humvees as they passed by.
More resistance hotspots
In Falluja, US occupation soldiers pounded two districts of the besieged town on Monday, local residents said.
Skirmishes in the Golan and Shuhada districts broke out around 11:20 (07:20 GMT) and were continuing.
Despairing at a largely-ignored truce, town elders have written to UN Secretary-General Kofi Annan asking him to mediate between resistance fighters and occupation forces.
"We are facing what
can be called ... war crimes, and the
situation can no longer bear the actions of the occupation forces who are behaving outside
all international laws"
Falluja truce delegation chief
They also urged Annan to set up an international investigation into the US-led occupation of Iraq.
Speaking on behalf of the Falluja town council, Muhammad Tariq Abd Allah said some of the council had "met with UN officials at the Amman office of the UN special envoy, Ross Mountain, and given them a message for the UN secretary general asking for his mediation.
"We told the UN people that if this mediation works, we are ready to ensure that the people of Falluja will turn in their heavy weapons and respect any agreement reached," he said.
Abd Allah stressed it was important the UN appoint a neutral mediator "who will give us guarantees that the Americans will also respect their end of the agreement".
Occupation war crimes
The Falluja delegation chief, Shaikh Muhammad Hamad al-Shihan, said he wanted the UN to know of the terrible hardship in Falluja since the occupation began.
"We are facing what can be called ... war crimes, and the situation can no longer bear the actions of the occupation forces who are behaving outside all international laws."
Hospital staff in Falluja cover the
dead body of a two-year-old boy
Al-Shihan also urged Annan to intervene quickly "personally to stop the bloodbath".
"The human rights violations that happened in Falluja are very serious and the massacres that happened there are unprecedented," he charged.
The US marines launched their offensive on 5 April after the murder of four US civilian contractors 50km west of Baghdad.
Iraq's interim health minister, Khudayr Abbas, said that by Thursday, 271 Iraqis had been killed in the city.
Iraqi burnt alive
ِِEarlier at the weekend, an Iraqi civilian was burnt alive in his vehicle after it came under fire near an occupation forces base in the city of Najaf.
Falah Hasan was found in his burning Datsun pickup truck near the base located between Kufa and Najaf. The base is manned by Spanish troops under Polish command.
A doctor at Kufa's Mid-Euphrates Hospital, Muhammad Abd al-Qasim, confirmed the 27-year-old's death. A military occupation spokesman said troops had had to be extra cautious following a mortar attack last Friday.