BAGHDAD - Two suicide car bombs exploded outside a joint U.S.-Iraqi military base near the northern town of Sinjar on Saturday, killing five people and wounding at least 45, a hospital official said.

Witnesses told the official the bombs exploded in quick succession at the entrance to the base, just south of Sinjar in the northwest of Iraq, close to the border with Syria.

Most of those wounded were laborers at the camp or Iraqi troops, they said.

Iraq's al Qaeda wing, led by Jordanian Abu Musab al-Zarqawi, said in an Internet statement it had carried out both attacks.

"A lion... drove a car filled with deadly mines and attacked the first checkpoint of a base in Sinjar housing crusaders and their agents," al Qaeda Organization for Holy War said in a statement posted on a Web site used by Islamists.

"Another martyr followed swiftly on his heels... and drove a water tanker which in fact was carrying fire. When he was in their midst and the masses gathered, he pressed the buttons."

The blasts followed a car bomb attack on an Iraqi police convoy in Tikrit late on Friday which killed two civilians and wounded 24 people, including nine police, Jalal Khoshi, a doctor at the local hospital said.

He said an ambulance driver carrying the wounded to hospital had also been shot and killed, but it was not immediately clear by whom.

South of Baghdad, outside the town of Hilla, unknown gunmen stopped a car carrying five Iraqi soldiers and opened fire, killing four of them and seriously wounding one, a spokesman for the Hilla police said.

There has been an upsurge in violence over the past month, with mostly Sunni Arab guerrillas stepping up their two-year-old insurgency since a new Shi'ite-led government was formed.

Nearly 700 Iraqis have been killed in car bomb blasts, ambushes and shootings since the beginning of May, while more than 60 U.S. troops have also died in the same period.

As well as the sustained high number of attacks on Iraqi security forces, there has been an increase in sectarian killings, with Sunni and Shi'ite Muslims dying in tit-for-tat attacks.