BAGHDAD, Iraq - US-led forces captured a senior al-Qaida leader who was responsible for hundreds of civilian deaths and housed foreign fighters who carried out suicide bombings, the US military said Wednesday.

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Meanwhile, the US-led coalition handed over security responsibilities in Iraq's Najaf province to Iraqi forces on Wednesday, a key step in troubled efforts to get the fragile government to stand on its own.

The leader, who was not identified, was arrested in a raid in Mosul on Dec. 14, the military said in a statement.

"The terrorist leader was attempting to flee from the location when Coalition Forces chased him across a street and detained him," the statement said.

It said the suspect served as al-Qaida's military chief in Mosul in 2005, and then took up the same job in western Baghdad.

"During that time, he coordinated car vehicle-borne improvised explosives device attacks and kidnap for ransom operations in Baghdad," the military said. It cited reports that said he organized an attempt to shoot down a US military helicopter in May this year.

"After a few months he fled Baghdad due to Coalition Forces closing in on him," the statement said.

The military said the capture would lead them closer to Abu Hamza al-Muhajir, also known as Abu Ayyub al-Masri, who took over as leader of al-Qaida in Iraq after his predecessor, Abu Musab al-Zarqawi, was killed in a US airstrike in June.

Mouwafak al-Rubaie, the Iraqi government's national security adviser, said this month that 60 percent of al-Qaida in Iraq's leadership has now been captured or killed.

Najaf was the third of Iraq's 18 provinces to come under local control, though US-led forces will remain on standby in case the security situation deteriorates. British troops handed over control of southern Muthana province in July, and the Italian military transferred Dhi Qar province to Iraqis in September.

"If we don't handle the responsibility, history will destroy us," Iraq's national security adviser, Mouwafak al-Rubaie, said at a ceremony in a stadium in the city of Najaf, the provincial capital south of Baghdad. He urged former members of Saddam Hussein's army to join Iraq's new military, and said he would do his best to get better equipment for forces nationwide.

The holiest city in Iraq for Shiites, Najaf is home to the iconic Imam Ali shrine near the city's huge cemetery - used by Shiites from throughout the country who come to the city to bury their dead.

It is a relatively peaceful area of Iraq, tightly controlled by police and Shiite guards.

A top US military said Iraqi forces in Najaf would get help if necessary.

"Coalition forces will continue to provide support if called upon," Maj. Gen. Kurt Cichowski said. "We will be quite literally just up the road."

Cichowski said more pilgrims would be able to enjoy religious sites in Najaf, with a safe airport and highways.

"Transferring responsibility is an indication of the increased capacity of the Iraqi police and the Iraqi army," he said.

Authorities imposed restrictions on driving in Najaf to ensure security for the handover event. At the ceremony, soldiers paraded around a soccer field, as donkeys pulled carts through the streets outside the stadium.

The US-led coalition wants to hand over control of police and all services to governors in Iraq's 18 provinces, and then pull back to larger bases. If violence developed that local police could not handle, a governor could request help from national police or the Iraqi army. To get coalition forces involved, a governor would need the permission of Iraq's prime minister.

While violence in Najaf is not as bad as other areas, much of it has been directed toward pilgrims. Attacks on houses of worship have stoked tensions between Shiite and Sunni Muslims, especially after the Feb. 22 bombing of a Shiite shrine in Samarra, north of Baghdad. The attack set off reprisals against Sunni mosques and clerics.

Najaf was the scene of heavy fighting in 2004 between the US Army and the militia of radical Shiite cleric Muqtada al-Sadr. Parts of the Shiite holy city lie in ruins.

Prime Minister Nouri al-Maliki, after meeting President Bush last month in Jordan, said the two leaders agreed on the need to speed up the Iraqi takeover of security responsibilities across the country. But questions remain about whether Iraqi forces can handle the task.

Even with US troops leading the battle, the Pentagon reported Monday that attacks on US and Iraqi troops and Iraqi civilians jumped sharply in recent months to the highest level since Iraq regained its sovereignty in June 2004.