BAGHDAD: ISIS has killed five policemen in a town near Iraq's biggest refinery, in an attack that may help ease pressure on some of its fighters trapped in the strategically important facility, a security official said Wednesday.

The official in a regional security command center said the insurgents mounted the operation at Tal Albu Jarad village as part of a battle for control of Baiji refinery, which has changed hands several times.

After receiving reinforcements, ISIS militants recaptured three neighborhoods in the town of Baiji near the refinery, but fresh clashes have erupted there, the security official said.

Iraq's U.S.-backed government is focused on two key areas in its bid to loosen the grip of ISIS, which controls a third of major oil producer Iraq and large parts of neighboring Syria.

As well as in the war of attrition at Baiji in the north, Iraqi government forces are hoping to oust ISIS in Anbar, a vast western province mostly held by the insurgents.