A U.S. soldier was sentenced to 25 years in prison for the murder of an Iraqi National Guardsman in May, the U.S. military said Saturday.

The announcement came as a spate of attacks involving insurgents and U.S.-led forces spread across Iraq. The U.S. military announced that four Marines were killed in three separate incidents in Anbar province Friday and that another soldier was killed by a roadside bomb Saturday in Baghdad, the capital.

In the restive city of Fallujah 35 miles west of Baghdad, U.S. warplanes launched airstrikes early Saturday and again at night. The attacks killed 15 people and wounded 30, doctors in the city told the Associated Press.

In Baghdad, gunmen fired on a vehicle carrying Iraqi National Guard applicants, killing six people, police told the Associated Press.

The conviction of Spec. Federico Merida was believed to be the first of a U.S. soldier in the murder of an Iraqi since the war began 18 months ago. The incident took place in May at a base near the city of Ad Dawr, near Tikrit, the statement said. Merida had pleaded guilty to the charges.

Courts-martial are open to the public, but the military did not announce Merida's case. He also received a dishonorable discharge and reduction in rank. No further details were available.

The fighting across the country illustrated the disparate battles that the U.S. military and the insurgents continue to wage in what has been an unrelenting wave of violence. In Fallujah, U.S. forces have launched what has become a prolonged bombing campaign to take back the city from loyalists of Abu Musab Zarqawi, a Jordanian-born militant linked to al Qaeda. Zarqawi has asserted responsibility for a string of recent kidnappings, including the videotaped beheadings this week of two hostages, Eugene "Jack" Armstrong and Jack Hensley, who were American contractors.

The fate of Kenneth Bigley, a British civil engineer who was kidnapped with the two Americans on Sept. 16, remained uncertain Saturday. Victoria Whitford, a spokeswoman for the British Embassy in Baghdad, said British officials were "not giving any credence" to a report that appeared on an Islamic Web site claiming that Bigley, 62, had also been beheaded.

The British government said it doubted the credibility of the statement because it appeared on a different Web site from one on which the beheadings of Armstrong and Hensley were originally posted. The statement also said insurgents had kidnapped seven British soldiers, which officials said they believed was untrue.

On Friday, the Muslim Council of Britain sent two negotiators, Daud Abdullah and Musharraf Hussain, to meet with religious leaders in Baghdad to try to win Bigley's release, according to the Reuters news agency. The group described the negotiators as "well-respected figures in the British Muslim community."

Another negotiator, Iqbal Sacranie, urged Bigley's captors to free him. "Our religion, Islam, does not allow us to harm the innocent," said Sacranie, the group's secretary general. He urged the kidnappers to "release this man back into the arms of his waiting family," Reuters reported.

In Cairo, an official for an Egyptian communications company, Orascom, said it had no plans to curtail operations following the kidnapping of six Egyptian engineers working for its subsidiary, Iraqna. The official said he had no information about the captives. "Our interest now is only getting them out alive," he said.

Family members of the captives pleaded for their safety. Radwa Mohammed Afify, wife of Mustafa Abdel Latif, one of the engineers, said she received a call from him Thursday night saying he was concerned about the security guards at his firm. She said he called again at 11 p.m. but did not speak into his phone. Rather, she heard shouting and voices.

"I kept calling his name in a loud voice, then the line died. It was then that I was sure he tried to tell me something. I kept trying to call him until the early morning and kept getting a message that the number is not available," she said in a brief interview. "I know that my husband will overcome his ordeal through faith and prayer."

Egypt's Foreign Ministry said it was trying to contact intermediaries in Iraq to win freedom for its citizens.

In addition to the kidnappings, insurgents have continued to attack police and National Guard recruits from the nascent Iraqi security forces, the linchpin of a U.S. strategy to hold nationwide elections in January. Hours after the attack on the National Guardsmen, four mortar rounds landed in a Baghdad sports club where hundreds of police recruits had been summoned for a meeting.

The fighting in Fallujah began at 11:30 p.m. Friday, when U.S. forces attacked what a U.S. military statement said was a "an offensive obstacle belt" composed primarily of concrete and earthen barriers containing bombs. Construction of the barriers was "considered a hostile act," the statement said, and served to "undermine and discredit the authority of Iraqi civic leaders and restrict the people of Fallujah from living a normal life."

Around 3:30 a.m. Saturday, insurgents operating out of vacant houses in the Askari neighborhood launched rocket-propelled grenades and mortars at a U.S. base on the periphery of the city.

At 4:05 a.m., witnesses said, U.S. warplanes and attack helicopters destroyed five houses that were used by the insurgents and another that was occupied by a family.

Rafid Hiyad Isawi, the director of Fallujah General Hospital, said a couple and their two children were among the dead. The U.S. military said in a statement: "There were no innocent civilians reported in the immediate area at the time of the strike."

Ahmed Zawbae said his brother Mahmoud and his family were killed in the raid. He said insurgents had warned the family to leave the area but his sister-in-law had been too ill to be moved.

"This area is turned into a war zone for both sides; this resulted in the killing of my brother and his family," Zawbae said. "They are civilians and innocent. What could they do?"

Correspondent Daniel Williams in Cairo and special correspondent Bassam Sebti contributed to this report.