The Islamic State group executed 150 civilians in western Iraq, where it is fighting to control the town of al-Baghdadi near a US airbase, a local official said Friday.

Malallah al-Oubeidi, the head of al-Baghdadi’s local council, said the victims were mostly members of the Albu Oubeid tribe, which is fighting alongside government troops against Islamic State.

According to al-Oubeidi, the 150 people were killed Thursday after the radicals seized them days earlier.

The central government in Baghdad did not comment.

Al-Baghdadi – located 5 kilometres from the Ain al-Asad airbase, where US Marines are training Iraqi soldiers – has been besieged by Islamic State for more than a week.

Last week, the base came under artillery and rocket fire by Islamic State forces.

Iraq’s top Shiite cleric, Ali al-Sisstani, on Friday urged the government to move to break the rebels’ siege of al-Baghdadi and prevent a “tragedy” among local residents.

Islamic State, an al-Qaeda splinter group, controls territory in western and northern Iraq.

Backed by Kurdish fighters and a US-led aerial campaign, Iraqi government troops have in recent months launched counterattacks to dislodge the radicals from the country.

A US military official said an operation to retake Mosul, Iraq’s second-largest city, from Islamic State fighters will begin in April or May if Iraqi and Kurdish forces are trained by then.

The offensive will involve 20,000 to 25,000 troops, a military official from US Central Command in Tampa, Florida, said Thursday.

The US military had previously said an offensive was being planned but had not given details.

Five Iraqi brigades will be trained ahead of the offensive, which will go forward only when the units are ready, the official said on the condition of anonymity.

From 1,000 to 2,000 Islamic State fighters are holding Mosul, which was captured in June when Iraqi troops laid down their arms and fled. It is strategically important because of its oil refineries.

The radical Sunni militia also controls considerable areas in neighbouring Syria, and there are reports it has established a foothold in chaotic Libya. SAPA