US forces launched a second wave of air strikes against the Islamic State extremists near Arbil in northern Iraq on Friday, destroying a militant convoy and killing a mortar team, the Pentagon said.

US forces launched a second wave of air strikes against the Islamic State extremists near Arbil in northern Iraq on Friday, destroying a militant convoy and killing a mortar team, the Pentagon said.

Shortly after 1400 GMT, US drones destroyed a mortar position and killed a group of militants. Just over an hour later four F/A-18 jets hit a seven-vehicle Islamic State convoy with eight laser-guided bombs.

"The US military continued to attack ISIL targets near Arbil today conducting two additional air strikes to help defend the city where US personnel are assisting the government of Iraq," Pentagon spokesman Rear Admiral John Kirby said.

President Barack Obama announced Thursday that he had authorized US air strikes to prevent fighters from the so-called Islamic State from attacking the capital of the Iraqi Kurdish region.

The strikes are also designed to break the siege of Mount Sinjar, where IS forces have cornered and reportedly threatened to kill thousands of civilian refugees from the Yazidi religious minority.

Overnight, a first strike saw an F/A-18 hit an IS artillery position with a 500lb bomb. Shortly beforehand US military planes had dropped food and water to the beleaguered Yazidis.

Friday's second wave saw unmanned drones brought into action.

"Remotely piloted aircraft struck a terrorist mortar position. When ISIL fighters returned to the site moments later, the terrorists were attacked again and successfully eliminated," Kirby said.

"At approximately 11:20 a.m. EDT, four F/A-18 aircraft successfully struck a stationary ISIL convoy of seven vehicles and a mortar position near Arbil," his statement continued.

"The aircraft executed two planned passes. On both runs, each aircraft dropped one laser guided bomb making a total of eight bombs dropped on target neutralizing the mortar and convoy."