Islamic State extremists stormed a police base in Iraq’s western Anbar province Monday in three explosive-laden Humvees set to detonate, killing at least 41 soldiers and Shiite militiamen, Iraqi officials say.

Police officials said the three suicide bombers simultaneously drove the military vehicles-- seized from the Iraq Army-- into the base in the Tharthar area, north of the ISIS-held provincial capital, Ramadi. The attack caused a large secondary explosion in an ammunition depot, the officials said. 41 died in the attacks and another 63 security forces were wounded.

Hospital officials confirmed the casualties. All officials spoke on condition of anonymity because they are not authorized to brief the media.

The attack resembled a massive, coordinated assault launched on Ramadi last month that allowed ISIS militants to capture the city, marking their biggest gain since a U.S.-led coalition began launching airstrikes against the extremist group last August. In that assault, the ISIS group also used Humvees looted from Iraqi security forces.

The loss of Ramadi prompted Iraqi Prime Minister Haider al-Abadi to order Shiite militiamen into the vast Sunni province, which was an insurgent hotbed during the eight-year U.S. intervention.

The Shiite militiamen have played a key role in pushing the Sunni ISIS group back elsewhere in Iraq, but have also been accused by rights groups of carrying out revenge attacks against Sunnis, charges denied by militia commanders.

Meanwhile, The United Nations mission to Iraq said Monday that more than 1,031 people were killed and another 1,648 were wounded in violence across the country last month.
  

The U.N. figures showed that 665 civilians and 366 members of the Iraqi security forces were killed in May. Baghdad was the worst affected province with 343 killed and 701 wounded. But, the UNAMI statement excluded deaths from ongoing fighting in Anbar, due to problems in verifying the "status of those killed." The figures also leave out insurgent deaths.

The Associated Press contributed to this report.