A year after the fall of Saddam Hussein US troops are battling gunmen in the old regime's Sunni heartland while unrest continues in Iraq's Shia cities.
Coalition troops now face gunmen from both Iraq's main communities
Mortar rounds appeared to signal the collapse of a truce called by marines in the Sunni city of Falluja where doctors counted 450 deaths this week.
UK Foreign Secretary Jack Straw said the coalition faced its "most serious" threat since the end of the war.
But the US military said operations against militants were "going well".
Brigadier General Mark Kimmitt said the operations would soon be over and added that the ceasefire in Falluja was still in effect while troops retained the right to defend themselves.
Rafi Hayad, director of Falluja's main hospital, told Reuters news agency that 450 Iraqis had been killed and 1,000 wounded in fighting since the US ringed the city of 300,000 to pursue the killers of four American security men ambushed there last week.
The US has announced the deaths of six more of its soldiers in combat over the past two days, bring to at least 39 the number killed since the Shia unrest erupted on Sunday.
Concern is growing over the situation in Falluja both within Iraq and abroad:
Russia has called on the sides in Iraq to show restraint and warned of "an impending humanitarian disaster" in the city
- A senior Sunni Muslim member of the US-appointed Iraqi Governing Council (IGC), Adnan Pachachi, called the operation illegal and completely unacceptable
- Kurdish IGC member Mahmoud Uthman said US policy was counter-productive
Muslim Friday prayers saw angry protests across the Middle East
The Iraqi interim Human Rights Minister, Abdel Basit Turki, and a member of the Iraqi Governing Council's rotating presidency, Iyad Allawi, both resigned on Friday without giving a reason for their decision.
Moqtada Sadr, the radical cleric leading unrest in Shia areas, has demanded the withdrawal of coalition troops from Iraq, saying US President George W Bush could no longer point to Saddam Hussein or weapons of mass destruction as reasons to be in Iraq.
President Bush has been consulting other coalition leaders by telephone.
A convoy of about 10 US fuel tankers was attacked and set on fire on Friday on the highway between Falluja and Baghdad, outside the town of Abu Ghraib.
According to an unconfirmed report by Reuters news agency, gunmen in Abu Ghraib have captured six foreign citizens, two of them "Americans".
Militants are already holding a number of foreigners hostage across Iraq and Palestinian leader Yasser Arafat has appealed for the release of two Palestinian aid workers among them.
In another incident, a Briton was shot dead whilst guarding electrical workers for a US company.
Japan is refusing to withdraw troops from Iraq despite the death threats hanging over three of its citizens taken hostage.
The US had called a truce in Falluja to allow for talks with local leaders and permit humanitarian supplies to enter the city but mortar and other fire punctured the air soon after Friday's noon (0800 GMT) start time.
IRAQ ONE YEAR ON
US military deaths since 'major combat over' on May 1 2003: At least 511
Other coalition deaths: At least 67
Total reported Iraqi civilian deaths: Estimates from 8,865 to 10,715 *
$33bn allocated so far for reconstruction
More than 200,000 recruited to Iraqi security forces
Oil exports up to 1.8m barrels a day, against 2m pre-war
Electricity back to pre-war levels
51 million new "Baath-free" textbooks in schools
170 newspapers in print
46 of 55 'most wanted' captured or killed, including Saddam
* Iraq Body Count, to 8/04/2004
** Coalition Provisional Authority
US marines backed by bombers and helicopter gunships have been coming under sniper and grenade attack.
There are reports of bodies in the streets and of makeshift clinics being set up, with medical supplies scarce.
Men, women and children are said to be trying to flee the city on foot through backstreets and paths which cut through fields.
Clashes with US troops are also reported in the mixed Sunni and Shia town of Baquba.
Coalition troops have restored control over the Shia city of Kut but fought new battles in the holy city of Karbala where at least three militants and an Iranian woman are said to have been killed.
In Baghdad, a mortar round fell near the Sheraton Hotel early on Friday evening, causing a loud explosion but no casualties.