US and Iraqi troops have entered Ramadi a day after anti-US fighters staged a show of force in the western city.

Operation Shank, involving 300 American Marines and 200 Iraqi soldiers, began on Friday, the latest in a series of operations in Anbar province, the heartland of anti-US fighting.

A US military statement said: "The purpose of the operation is to disrupt a terrorist group that utilises an area of Ramadi as its base for attacks on local Ramadi citizens, Iraqi and US military."

Fighters launched a brief assault in Ramadi on Thursday, firing mortar rounds and rockets at a US base and local government buildings.

But after a couple of hours, most of the fighters dispersed and the city appeared to return to relative calm.

The US military said reports of fighters taking control of the town were "completely unsubstantiated".

Town residents said there was no sign of the fighters on the streets on Friday.

US marines killed

A roadside bomb near the Iraqi city of Fallujah killed 10 U.S. Marines Friday and wounded 11 others, the deadliest attack on the occupation forces in recent months, the Marine Corps reported.

According to The Associated Press report, the Marines, killed in the roadside bomb, were from Regimental Combat Team 8, of the 2nd Marine Division.

In line with usual military practice, the names of killed Marines, who were on a foot patrol outside of Fallujah, were withheld pending notification of their relatives.

Threat to kill Iraq hostages

Aljazeera has aired a new videotape in which kidnappers of four Christian peace activists in Iraq have threatened to kill their hostages unless all prisoners in US and Iraqi detention centres are released.

The captors gave the two governments until 8 December to meet their demands, Aljazeera quoted a statement delivered with the tape aired on Friday.

Two Canadians, one American and a Briton are being held.

The Canadians were shown eating from plates of what appeared to be Arabic sweets. The Briton and American hostages were shown talking to the camera but no sound was transmitted.

The two were calling on the US and British governments to withdraw from Iraq, Aljazeera said, quoting the kidnappers' statement.

Meanwhile, the mother and sister of a German woman taken hostage in Iraq have called on her kidnappers to show mercy and release her in an appeal shown on Aljazeera on Friday.

Osthoff, a 43-year-old archaeologist, disappeared a week ago. Earlier this week, her kidnappers said in a videotaped message that they would kill her if Germany did not end all support for the Iraqi government.

Germany helps train Iraqi forces outside the country but has ruled out sending troops there.

An image from the tape, delivered to Germany's ARD public television in Baghdad, showed what appeared to be Osthoff and her driver sitting on the ground surrounded by three armed, masked men.

"We appeal to you to spare the lives of my innocent sister and her escort," her sister Anja Osthoff said. "My sister has lived for a long time in your country and is devoted to it. She brought sick people medicine. She loves Iraq's great culture."

It was not clear who had abducted the archaeologist, a converted Muslim who had spent about 15 years working on excavations in Iraq before UN sanctions forced foreign experts out of the country in the late 1980s.

PHOTO CAPTION

U.S. Navy Corpsman looks at an Iraqi boy looking at his gun in Saadah, Iraq, eight miles from Syria, Friday, Dec. 2, 2005. (AP)