US forces Saturday launched more airstrikes in northern Iraq to defend attacks on Yazidi civilians, the Pentagon said, on the second day of its military campaign.

President Barack Obama had announced this week he had authorized US air strikes in part to help break the siege of Mount Sinjar, where fighters from the so-called Islamic State forces have cornered and reportedly threatened to kill thousands of civilian refugees from the Yazidi religious minority.

The first strike, at around 11:20 Washington time (1520 GMT), was carried out by a mix of fighter jets and drones, the United States Central Command, which covers the Middle East, said in a statement.

It targeted two armored personnel carriers firing on Yazidi civilians near Sinjar, the state said, adding one of the two IS vehicles was hit and destroyed.

Twenty minutes later, after monitoring the second personnel carrier, the US aircraft struck and apparently destroyed two additional armored personnel carriers and an armed truck.

And at around 3:00 pm Washington time (1900 GMT), a US plane located, struck, and apparently destroyed another armored personal carrier near Sinjar.

On Friday, two waves of airstrikes hit an IS artillery position, destroyed a militant convoy and killed a mortar team.

US and Iraqi aircraft have also air dropped food and water to the thousands of mainly Yazidis stranded on Mount Sinjar since fleeing IS attacks on their homes a week ago.

Obama said earlier Saturday he viewed the US offensive in Iraq as a “long-term project” to rout out militants and deliver aid to beleaguered civilians.

He has also said US air strikes aim to prevent IS fighters from attacking the capital of the Iraqi Kurdish region, where the US has a diplomatic mission and military advisers.

“I’m not going to give a particular timetable, because as I’ve said from the start, wherever and whenever US personnel and facilities are threatened, it’s my obligation, my responsibility as commander in chief, to make sure they are protected,” Obama told reporters.

The US president has justified the intervention by warning of the risk of genocide against the small Yazidi minority.

However, he has repeatedly vowed he would not send US combat troops in the first American offensive since Washington pulled out its forces in 2011 after nearly a decade of brutal war.