Arbil (Iraq) (AFP) - US jets struck jihadist positions in northern Iraq on Friday, a potential turning point in a two-month crisis Washington said was threatening to result in genocide and to expose US assets.

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

Pentagon spokesman Rear Admiral John Kirby said on Twitter that US forces bombed the jihadists after artillery fire against Kurdish regional government forces defending their capital Arbil.

Two US jets hit a mobile artillery piece, he said. A Kurdish official said the strikes targeted the towns of Gwer and Makhmur, southeast of the autonomous Kurdish region.

The US operation began with air drops of food and water for thousands of people hiding from the Sunni extremist militants 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.

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

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

Panic had begun to grip Arbil after IS thrust into the Nineveh plains separating the city from the main jihadist of Mosul and Obama's decision was welcomed there.

"We were very nervous these past few days. Daash (Islamic State) is powerful and well-equipped," said Karwan Ahmed, 27, a taxi driver. "This is good news."

The Kurdish peshmerga, short of ammunition and stretched thin along a huge front, have been forced to retreat in the face of brazen assaults by the jihadists.

- Captured dam -

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.

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Obama's announcement came after an emergency UN Security Council meeting called by France, which also offered to support the emergency effort.

The capture of Mosul dam was another setback for the peshmerga who had been defending it and gave jihadists a power of life and death over a huge swathe of land.

A Kurdish and a local official said jihadists took it over on Thursday and warned that any "unscientific manipulation" could have disastrous consequences.

A 2007 letter to the Iraqi government based on a US assessment had warned that "catastrophic failure of Mosul dam would result in flooding along the Tigris river all the way to Baghdad."

While IS has weaponised dams before, Mosul dam provides water and electricity to its main stronghold and is crucial to its own state-building efforts.

In Mosul, the country's second city, identical sermons were delivered in all mosques Friday, residents said, with worshippers asked to swear obedience to the "caliph", Abu Bakr al-Baghdadi.

- Stranded Yazidis -

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."

Iraq's military chief of staff told AFP he expected to see his forces and the peshmerga reclaim large tracts of land "in the coming hours".

He said he thought the US air strikes would extend to other towns controlled by IS but he did not say which ones. The jihadists control towns west, south and north of Baghdad.

There has been daily fighting on several fronts for two months and as the conflict escalated Friday, the US banned its civilian airliners from overflying Iraq, Britain asked its nationals in parts of Kurdistan to leave and global stock markets were shaky.

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 jihadists, who in late June proclaimed a "caliphate" straddling Syria and Iraq, has brought a country already rife with sectarian tension closer to collapse.

Many people in Baghdad were sceptical of Obama's motives.

"He did nothing for three years but something happened to the Kurds and the Christians and he started talking about terrorism," said Rashaad Khodhr Abbas, a retired civil servant.

"Where were you all this time, Obama?"

IS has enjoyed a spectacular run of military successes in Iraq, but the group also scored a key victory in Syria, with the capture overnight of a key army base in Raqa province.

Observers say one of the main obstacles to coordinated action by all of IS's Iraqi foes is Prime Minister Nuri al-Maliki, accused by many of having institutionalised sectarianism in recent years.

The United States, the Shiite religious leadership, powerful neighbour Iran and even much of his own party have all pulled their support but Maliki has dug his heels in.