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.