A bomb planted between a Sunni mosque and a youth center exploded during Friday prayers, killing four people and wounding another nine, police said.

In other violence, gunmen in Tikrit killed two civilians who were employed by U.S. troops, while a U.S. Marine was killed in action in western Iraq, officials said.

The Baghdad blast hit just as worshippers began leaving the al-Ali al-Aadhim mosque in a southeastern neighborhood, said police Capt. Ali Mahdi.

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The attack came during a four-hour driving ban police hoped would hold down sectarian attacks that have threatened to divide the capital city in recent weeks.

The Marine was assigned to Regimental Combat Team 5, which operates in Anbar province, and died Thursday, the command said.

At least 2,570 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

The two civilians were killed by the gunmen in a drive-by shooting in the al-Qadissya neighborhood of Tikrit, about 80 miles north of Baghdad, police said.

Gunmen in the nearby town of Beiji also killed a man who worked for a railroad company.

Elsewhere, a bomb tore through a pipeline southwest of Samarra, about 60 miles north of Baghdad. The pipeline carries oil from Iraq's largest refinery in Beiji to Baghdad's Dora neighborhood, police said.

A rocket also exploded in downtown Baghdad at midmorning, injuring three, police said. The attack came during a four-hour driving ban police hoped would hold down violence during Friday prayers.

Iraq's most influential Shiite political leader, meanwhile, said the country will achieve stability only if its own security agencies take full control with no meddling in their work.

Addressing a large crowd in the southern holy city of Najaf, Abdul-Aziz al-Hakim, head of the Supreme Council of the Islamic Revolution in Iraq, said the unstable security situation was due to "being lax in hunting down terrorists and upholding wrong policies of dealing with them."

Al-Hakim has long complained that the Americans should turn over more responsibility for security to the Iraqis and give them greater latitude. U.S. officials last year complained that the Shiite-dominated security services were trampling human rights and further alienating the Sunni minority.

"Iraq will not go back to normal except by achieving comprehensive security," al-Hakim said, listing a number of prerequisites for that, including "turning over the security file to the Iraqi apparatus and stopping the interference in their work."

The Shiite politician, whose group SCIRI has ties with Iran, said the government should also expel "the terrorist organizations present in Iraq," in particular the People's Mujahedeen, an Iranian resistance group.

The organization fled to Iraq in the early 1980s after it fell out with the clerical regime of Ayatollah Ruhollah Khomeini and continues to oppose the current Iranian regime. During Saddam Hussein's rule, the movement used Iraq as a base for operations against Iran's government.

Al-Hakim's speech marked the third anniversary of the death of his elder half brother, Mohammed Baqr al-Hakim, who was assassinated in 2003 in an al-Qaida linked car bombing. The elder brother was a revered ayatollah and leader of SCIRI. Both lived in exile in Iran under Saddam and had returned soon after the U.S.-led invasion.