A British engineer abducted with two American contractors three weeks ago was beheaded by his captors, according to a videotape of the slaying and comments from the victim's family.

The graphic video showing the slaying of Kenneth Bigley, 62, followed the same pattern as the beheadings of his colleagues, Eugene "Jack" Armstrong of Hillsdale, Mich., and Jack Hensley of Marietta, Ga., on Sept. 20 and Sept. 21. Bigley was shown seated in an orange jumpsuit before six black-clad insurgents of Monotheism and Jihad, the organization headed by the Jordanian militant Abu Musab Zarqawi, the Reuters news agency reported.

After reading a statement, one of the insurgents is seen drawing a knife from his belt and cutting off Bigley's head while three others hold him down. The group said the British government had failed to meet its demand to release all female prisoners held by occupying forces at two military prisons in Iraq.

"We can confirm that the family has received absolute proof that Ken Bigley was executed by his captors," Bigley's family said in a statement released by the British Foreign Office.

"We will always remember Ken for his love, compassion and, above all, his 'Liverpool' sense of humor," the statement said. "He was a truly wonderful father, husband, brother and son. The loss to our family . . . is immeasurable. The horror of these final days will haunt us forever."

In London, Foreign Secretary Jack Straw disclosed that the British Embassy in Baghdad had been approached four days ago by an unidentified intermediary and that messages had been exchanged with Bigley's abductors. But Straw said the captors had stuck to their demands that all female Iraqi prisoners be released, even though British authorities had no authority over the two women who remain in custody. Straw said the government had adhered to its policy of not negotiating with the captors.

"We believed, and still believe, that the policy we set out was the right one," he said at a news conference. Still, he insisted, "we did everything that we possibly could."

After the two Americans were killed, videos posted on the Internet showed Bigley pleading with Prime Minister Tony Blair to agree to the kidnappers' demands. On Friday, however, Bigley's family said it believed that "our government did all it possibly could to secure the release of Ken in this impossible situation. It could be that the fate of Ken, Eugene and Jack was sealed from Day One. We will never know."

However, one of Bigley's brothers, Paul, wrote that Blair had "blood on his hands" in a statement to the Stop the War Coalition, a British group that opposes the conflict, news agencies reported.

Blair told reporters in London that he felt "utter revulsion at the people who did this, not just at the barbaric nature of the killing, but the way, frankly, they have played with him over the past few weeks."

Iraq's defense minister, meanwhile, said the country's interim government had reached an agreement that would enable Iraqi forces to enter the rebel-held city of Fallujah, where U.S. military officials say Zarqawi is based. The accord, struck with civic leaders in Fallujah, calls for insurgents there to give up heavy weapons and mandates a three-day halt to airstrikes there by U.S. forces, Reuters reported.

"The agreement includes a cease-fire on strikes on Fallujah for three days initially, followed by the entry of Iraqi forces without any intervention from multinational forces," Defense Minister Hazim Shalan told the London-based Asharq al-Awsat newspaper. "The city's elders and men have welcomed this initiative, and now we are studying the technical steps to enter."

A U.S. airstrike on Fallujah early Friday killed 11 Iraqis and wounded 17, according to Reuters. A physician in the city said that a groom out on his wedding night was among the dead and that his new wife was among the wounded.

The U.S. military said in a statement that the "precision strike," which occurred at 1:15 a.m., targeted a safe house Zarqawi's associates used to plan operations. The statement did not mention casualties but said that the bombing campaign in Fallujah has involved more than a dozen airstrikes since September and that "a large percentage" of the leaders of Zarqawi's group had been "killed or captured following an intense and relentless campaign."

Negotiations to disband the militia loyal to the Shiite Muslim cleric Moqtada Sadr also appeared to be progressing.

Iraq's interim minister for national security, Qasim Dawood, issued a statement welcoming Sadr's "announcement . . . that his militia will disband their weapons, respect the authority and unit of the state and abide by the rule of law in Iraq."

Sadr has not made such a promise publicly, a significant omission given the personal loyalty he commands among many of his fighters, who also revere the memory of his father, an esteemed grand ayatollah.

Correspondent Glenn Frankel in London and special correspondents Bassam Sebti and Khalid Saffar in Baghdad contributed to this report.