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. […]