BAGHDAD (AP) — Islamic State group militants armed with a rocket launcher shot down an Iraqi military attack helicopter Friday in the country’s north, authorities said, highlighting their ability attack aircraft as a U.S.-led coalition expands its efforts to combat the extremists.

Fighters downed the Mi-35 helicopter between the towns of Beiji and al-Senniyah in northern Iraq, an official with the Iraqi Defense Ministry said. An official with the Iraqi air force corroborated the information, saying the helicopter’s pilot and co-pilot were killed in the crash.

Beiji, located 130 miles north of Baghdad, is home to Iraq’s biggest oil refinery.

Both officials spoke on condition of anonymity as they are not authorized to speak with journalists.

Militants with Islamic State group have taken over parts of western and northern Iraq this year, causing the Iraqi military to fracture while tensions intensified between Muslim sects. When fighters conquered Mosul, Iraq’s second-largest city, in June, the U.S.-trained Iraqi military crumbled, and the militants seized tanks and other military equipment, helping to steamroll their onslaught.

The shoot down of the helicopter shows the Islamic State group’s ability to counter air operations, potentially putting at risk U.S.-led airstrikes in the country. U.S. Central Command said it had carried out airstrikes in Sinjar and Fallujah over Thursday and Friday.

Iraq’s Prime Minister Haider al-Abadi has embarked on a plan to restructure the military and vowed to create a national guard that will defend the country and protect civilians without promoting sectarianism.

Iraq’s most revered Shiite cleric called on al-Abadi to create a national security force that doesn’t promote sectarianism at a time of heightened tensions between ethnic and religious groups.

The national guard, proposed by al-Abadi, should be a force of “patriotism and purity” to help pull the country out of the crisis, the reclusive al-Sistani said in his Friday sermon delivered by his spokesman Abdul Mehdi Karbalaie in the city of Karbala.

There must be “careful selection with regard to sectarianism or ethnicity or national building within the national guard, so that it does not generate feelings among the enrollees that they are defending certain sects,” al-Sistani said.

An Iraqi Cabinet also was selected on Sept. 8, with the exception of the key posts of the defense and interior ministers, with lawmakers failing to agree on who should be nominated. Al-Sistani called upon al-Abadi to select candidates for these critical roles following the Islamic Eid al-Adha holiday next week.

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