Islamic State fighters stormed a town in Iraq's western Anbar province on Saturday, killing at least 19 policemen and trapping others inside their headquarters, in the latest attack in the desert region where it controls large amounts of territory, officials said.

Islamic State seized the town of al-Wafa, 45 km (27 miles) west of Anbar's capital Ramadi on Saturday after starting its assault early on Friday.

With the capture of al-Wafa, Islamic State now controls three major towns to the west of Ramadi, including Hit and Kubaisa. Islamic State and government forces have been bogged down in a months-long battle for Ramadi.

Al-Wafa fell in a surprise attack that drew fresh attention to the Iraqi government's struggle to arm Sunni tribes in western Iraq who are fighting Islamic State.

"Police forces have been fighting Islamic State fighters since Friday, but lack of ammunition forced it to retreat and losing the town. I'm frustrated because we were left alone without support," said Hussain Kassar, the town's mayor.

Police forces backed by few members of government-paid Sunni tribal fighters tried to prevent the militants from crossing the sand barrier surrounding the town, but were overwhelmed when sleeper cells from inside open fired on them, the mayor and a police officer said.

Police forces and the pro-government Sunni fighters were forced to retreat to a nearby police-brigade headquarters bordering their town.

"We are trapped inside the police 18th brigade. Islamic State managed to surround us today. If no government forces were sent to help us then we will be exterminated," the mayor, who was with the police forces that withdrew from al-Wafa, said by telephone.

Elsewhere in western Anbar, Islamic State militants executed at least 21 Sunni tribal fighters on Friday after capturing them near al-Baghdadi town on Wednesday, local officials and tribesmen said on Saturday. Islamic State has besieged al-Baghdadi, also to the west of Ramadi, since October.

All the bodies had bullet wounds to the head and chest and were dumped inside an orchard near the Islamic-State controlled town of Kubaisa.

The radical Sunni Muslim militants have captured swathes of western and northern Iraq, including the north's biggest city, Mosul, in June. They now hold large territory from western Anbar and Nineveh provinces that extends across the border into Syria.