Tony Blair is to fly out to Washington next week to meet with US President George W Bush amid growing instability in Iraq.
As many as a dozen US Marines were killed when they were attacked in the Iraqi city of Ramadi near the Sunni hotbed of Fallujah, a US defence official said.
The official said initial reports indicated that dozens of Iraqis assaulted the Marine position near the governor's palace in Ramadi.
"There may have been as many as a dozen Marine deaths," the official said, adding that "a significant number" of Iraqis were killed.
Amid speculation that the country is descending into civil war, Iraq's Shia majority has now joined the Sunni Muslims in attacking coalition forces in a spate of violent attacks over the past few days.
Mr Blair held talks with Iraqi Foreign Minister Mr Hoshyar Zebari after a radical Shi'ite group, led by the firebrand cleric Moqtada al-Sadr, turned to violence in Baghdad.
US officials have issued a warrant for al-Sadr's arrest, while Mr Bush has insisted that the deadline for the transfer of political power from the US to Iraq should remain as June 30.
But the recent unrest in Iraq has raised questions about whether the hand-over date is realistic.
Democrat presidential candidate John Kerry suggested Bush may have chosen June 30 for political reasons.
He said: "I have always said consistently that it is a mistake to set an arbitrary date and I hope that the date has nothing to do with the election here in the United States."
Meanwhile, former UN weapons inspector Hans Blix has said the costs of the war in Iraq have outweighed the benefits of removing Saddam Hussein.
"Bush declared war as a part of the US war on terror, but instead of limiting the effects of terror, the war has laid the foundation for even more terror," Blix said.
Apr 4: 29 die in Iraqi clashes
