Militants ambushed a convoy of 60 tanker trucks heading to Baghdad from Iraq's largest refinery Wednesday, destroying four of the vehicles and damaging 15 others, police said.

The attack, by insurgents with rocket-propelled grenades and machine guns, came after the Beiji refinery had reopened Sunday after being closed since Dec. 18 because of threats against tanker drivers. The drivers began carrying fuel again this week after the government promised extra protection for the convoys.

Three Iraqi army vehicles, which had been guarding the convoy, were also destroyed in the attack about 25 miles north of Baghdad, police Lt. Abdul Zahra Qassim said.

A militant group, the Islamic army in Iraq, claimed responsibility. The claim was posted on an Internet site commonly used by militant groups and could not be independently verified.

The claim said the group attacked "a convoy of tanker trucks and managed to burn and destroy 20 tankers. This was followed by the destruction of troops who were sent to rescue the tankers at Mishahida."

Iraq's Cabinet said Monday the Beiji refinery had resumed supplies to Baghdad and other cities after the Iraqi army sent more troops to guard the tanker trucks.

The Beiji closure contributed to a fuel crisis caused by an acute shortage due to Iraq's decreased refinery capacity. The crisis was compounded by government decision Dec. 18 to increase fuel prices and abolish some subsidies for oil products.

The area near Beiji, 155 miles north of Baghdad, has in the past been a hotbed of the insurgency. The town is near Tikrit, Saddam Hussein's hometown and the region where the former leader was arrested two years ago.

At least one tanker was attacked with a roadside bomb earlier in the day just outside Tikrit and AP Television News video showed it awash in flames.

On Tuesday , U.S. aircraft bombed a building near Beiji where suspected insurgents were hiding, killing seven people.

The U.S. military said an unmanned aircraft spotted three people planting a roadside bomb Monday night near the town and that Navy F-14s were called in. The three left the road site and were traced to a nearby building — which was destroyed.

It was unclear if Wednesday's attack on the tankers would again lead to a closure of the Beiji refinery.

Price increases and shortages last week led to the resignation of the oil minister, protests in many cities and riots in oil-rich Kirkuk, where police shot and killed four people Sunday as demonstrators burned down a fuel station.

Iraq oil exports have fared no better, and in December hit their lowest ever level since the war. Only 34.4 million barrels were exported last month, or about 1.1 million barrels per day — the lowest average since Iraq resumed exports after the U.S.-led invasion in March 2003.

Almost all the oil was exported from Iraq's southern oil terminals because of continuing sabotage of the country's northern oil facilities.