Updated

Militants shot an American hostage in the back of the head saying they killed the soldier because of U.S. policy in Iraq, Al-Jazeera (search) television said Tuesday, hours after Washington transferred sovereignty in Iraq to an interim government.

The Arab-language station reported that the slain soldier was Spc. Matt Maupin (search), but the U.S. military said it could not immediately confirm whether a man shown being shot in a murky videotape was indeed Maupin, who was taken hostage after an April 9 attack outside Baghdad.

The report did not say when Maupin, 20, of Batavia, Ohio, was killed.

Monday's surprise transfer of sovereignty came two days earlier in an apparent attempt to foil the timing of expected attacks by anti-American insurgents intent at undermining the transfer.

There were no major attacks throughout the day. But after nightfall Monday, four heavy explosions rang out in central Baghdad, near the U.S.-held Green Zone — a near daily occurrence in the capital. The military said there were no injuries in the blasts, which were caused by mortar fire.

On Tuesday, a roadside bomb exploded as a senior Kurdish police official was heading to work, killing one of this guards and wounding him, police said.

Maj. Ahmed al-Hamawandi (search), the head of police in the Kurdish district of Azadi in Kirkuk, suffered minor injuries in the attack that occurred at around 8:50 a.m., said police Col. Sarhat Qader.

Sectarian tension has been on the rise in Kirkuk, a city that sits atop vast oil reserves, and Kurdish officials and police have been the frequent target of attacks by gunmen.

The U.S. civilian authority, which rode in on a swift military victory that swept away Saddam's generation-long regime, withdrew quietly on Monday. Its leader, L. Paul Bremer (search), left Iraq aboard a military plane two hours after the transfer and was swiftly succeeded by U.S. Ambassador John Negroponte (search).

Hours later, NATO leaders agreed to help train Iraq's armed forces — a decision that fell short of U.S. hopes that the security alliance would take a larger role in Iraq.

Iraq's tentative step toward democratic rule will operate under major restrictions — some imposed at the behest of the country's influential Shiite Muslim clergy, which wanted to limit the powers of an unelected administration.

The interim government will hold power for seven months until, by U.N. Security Council resolution, elections are held "in no case later than" Jan. 31. The Americans retain responsibility for security.

Bush raised no objection to Iyad Allawi's possibly imposing martial law in Iraq or taking hard-line measures to deal with militants such as Abu Musab al-Zarqawi, the most wanted man in the country.

"He may take tough security measures to deal with Zarqawi, but he may have to," Bush said. "Zarqawi is the guy who beheads people on TV. He's the person that orders suiciders to kill women and children."

Al-Jazeera aired a video showing a blindfolded man identified as Maupin sitting on the ground. Al-Jazeera said that in the next scene, gunmen shoot the man in the back of the head, in front of a hole dug in the ground. The station did not broadcast the killing.

Maj. Willie Harris, spokesman for the Army's 88th Regional Readiness Command, said the man in the footage could not be clearly identified but that the videotape is being analyzed by the Department of Defense.

"There is no confirmation at this time, that the tape contains footage of Matt Maupin or any other Army soldier," he said, adding that the Maupin family was briefed "as to the existence of a videotape."

Al-Jazeera said a statement was issued with the video in the name of a group calling itself "The Sharp Sword against the Enemies of God and His Prophet."

In the statement, the militants said they killed the soldier because the United States did not change its policies in Iraq and to avenge "martyrs" in Iraq, Saudi Arabia and Algeria.

Maupin was among nine Americans, seven of them contractors, who disappeared after an ambush on a convoy west of Baghdad on April 9.

The bodies of four civilian employees of Kellogg Brown & Root — a subsidiary of Vice President Dick Cheney's former company Halliburton — were later found in a shallow grave near the site of the attack. The body of Sgt. Elmer Krause, of Greensboro, N.C. was later found.

One civilian driver, Thomas Hamill of Macon, Miss., was kidnapped but escaped from his captors nearly a month later. The others are missing.

In a separate hostage-taking, the father of a U.S. Marine who was reported kidnapped by militants on Monday issued a plea for his release. The captors of Cpl. Wassef Ali Hassoun have threatened to behead him.

Four other hostages — three Turks and a Pakistani — all face threats of beheading in the next two days in a new flurry of abductions and death threats in Iraq.

Maupin appeared days after the attack in a video showing him sitting on the ground in front of armed militants. There had been no word on his fate since.

Maupin — who was assigned to the Army Reserve's 724th Transportation Company, based at Bartonville, Ill. — was promoted in absentia on May 1 from private first class to the rank of specialist, said Maj. Mark Magalski, a spokesman for the 633rd QM Ballation, based in Cincinnati.

His abduction came amid a wave of kidnappings in which dozens of foreigners were snatched. Most were later freed, though an Italian and a Lebanese man were killed.

More recently, the abductions have taken a more grisly turn with the kidnapping and subsequent beheadings of American Nicholas Berg last month and South Korean Kim Sun-Il last week.

Hassoun, an American Marine of Lebanese descent, was shown blindfolded, with a sword brandished over his head in a videotape aired on Al-Jazeera on Sunday. The militants threatened to behead him unless all Iraqis "in occupation jails" are freed. They did not set a timeframe.

"I appeal to the kidnappers and to their conscience and faith to release my son," his father, Ali Hassoun, said in an interview with The Associated Press at his house in the northern Lebanese port city of Tripoli.

"He is not a fighter. I hope that they will respond favorably to my appeal. May God reward them," he said.

His kidnappers identified themselves as part of "Islamic Response," the security wing of the "National Islamic Resistance — 1920 Revolution Brigades." The name refers to the uprising against the British after World War I.