Four U.S. Marines have been killed in an attack west of the Iraqi capital, as a deadline imposed by Muslim militants threatening to execute a South Korean hostage expires.

Suspected insurgents killed four U.S. Marines in the Sunni Muslim stronghold of Ramadi. Speaking with reporters, Brigadier General Mark Kimmitt said commanders were alerted when the soldiers failed to report in to their superiors from the field.

"We sent a quick reaction force to their location," he said. "I can confirm that we do, in fact, have four servicemen dead."

News of the deaths came as a deadline was set to expire late Monday for a South Korean hostage. Muslim militants threatened to kill the businessman unless South Korea reverses a decision to dispatch 3,000 troops to northern Iraq, a demand South Korea has rejected.

A videotape that surfaced Sunday shows the businessman Kim Sun-il pleading for his life. For more than a year, he has worked for a Korean firm that is helping supply the U.S. military in Iraq. Coalition officials told reporters that efforts to locate and rescue the hostage were continuing.

Meanwhile, a military judge has ruled that top U.S. commanders in Iraq may face questioning from attorneys representing servicemen accused of abuse at the Abu Ghraib prison outside Baghdad. Among those who may be questioned are Lieutenant General Ricardo Sanchez, the head of U.S.-led forces in Iraq, and General John Abizaid, the commander for the region.

The judge also ruled that Abu Ghraib be preserved, as least for now, as a crime scene, and not destroyed as the Bush administration has suggested.

Lawyers for two defendants say, in addition to U.S. commanders in Iraq, they want to question Defense Secretary Donald Rumsfeld. Attorney Paul Bergin said "the individuals who are responsible for what happened in this particular case, that is high level individuals within the United States government, hopefully will be brought to justice, but the scapegoats in this case will be set free."

The Bush administration maintains that at no time did it authorize interrogation tactics of prisoners that violated U.S. law or international conventions.

Elsewhere, the flow of oil has been restored to Iraqi sea terminals after the repair of a pipeline severed in last week's blast by saboteurs.