US fighter jets have bombed positions of self-declared jihadists in northern Iraq, in what the federal and Kurdish governments vowed would allow them to start clawing back areas lost in two months of conflict.

President Barack Obama's order for the first air strikes on Iraq since he put an end to US occupation in 2011 came after fighters from the Islamic State group made massive gains on the ground, seizing a dam and forcing a mass exodus of religious minorities.

The Pentagon on Friday said US forces bombed an artillery position after the Islamic State attacked Kurdish regional government forces who are defending their capital Erbil.

Hours later, it said a drone destroyed a mortar position and jets hit a seven-vehicle convoy belonging to the Islamic State with eight laser-guided bombs.

The US operation began with air drops of food and water for thousands of people hiding from the group in a barren northern mountain range.

Many people who have been cowering in the Sinjar mountains for five days in searing heat and with no supplies are Yazidis, a minority that follows a 4,000-year-old faith.

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Late Friday, the Pentagon said that cargo planes escorted by combat jets made a second air drop of food and water to "thousands of Iraqi citizens" threatened by the fighters on Sinjar mountain.

Obama accused the Islamic State group, which calls Yazidis "devil-worshippers", of attempting "the systematic destruction of the entire people, which would constitute genocide".

Strikes 'limited in scope'

The UN said it was "urgently preparing a humanitarian corridor".

Kurdish peshmerga forces, short of ammunition and stretched thin along a huge front, had been forced to retreat in the face of brazen assaults by the Islamic State.

Their withdrawal from the Christian heartland on Wednesday and Thursday sparked a mass exodus - 100,000 people according to Iraq's Chaldean patriarch - and spurred Western powers into action.

"Fighters captured US-made weapons as Kurdish troops withdrew from various regions. Washington also wants to address that," Al Jazeera’s Jane Arraf, reporting from Erbil, said.

Obama suggested the strikes would be "limited" in scope. But he "has not laid a specific end date", White House spokesman Josh Earnest told reporters, while insisting a "prolonged military conflict that includes US involvement is not on the table".

Obama came to office determined to end US military involvement in Iraq, and in his first term oversaw the withdrawal of the huge ground force deployed there since the 2003 American-led invasion.

But the capture of huge swathes of land by the Islamic State group, who in late June proclaimed a "caliphate" straddling Syria and Iraq, has brought a country already rife with sectarian tension closer to collapse.