The moment the Kurds had been waiting for arrived in mid-afternoon; two US jets streaked low through a hazy sky dropping precision bombs on an artillery piece that Islamic State (Isis) militants had been using to attack Irbil.

After a long lull the air strikes intensified into the evening – though it was not clear who carried them out – muting Kurdish officials' concerns that the US return to Iraq would be little more than symbolic and Isis's momentum might only be slowed temporarily.

The provenance of the jets bombing south of Irbil on Friday evening could not immediately be established. The US has only acknowledged carrying out one attack. Kurdish officials suggested France may have been responsible, although French officials have not acknowledged sending fighter jets into combat.

The regular thud of bombs into the northern plains did not, however, slow the exodus from Arab Iraq into what is fast becoming the crumbling country's last redoubt. Tens of thousands of Iraq's newest displaced poured into the Kurdish north on Friday adding to the estimated half a million arrivals now being hosted by the regional administration.

Many of those arriving in Irbil were not as reassured as Kurdish officials by the return of the US to the battlefield. "We lost everything in an afternoon," said Miriam Athous, a Christian woman from Tal Kaif, south-west of Irbil. "Why should we be happy that the Americans come now? We were sitting in our homes like sheep in a lion's den for two months."

For all its order and new money, Irbil remained a city ill at ease on Friday. Air strikes had stopped Isis on the outskirts, but elsewhere the jihadist group remained ascendant. US officials said Isis had seized the strategic Mosul Dam from Kurdish peshmerga forces. The fight for the dam has been bitter and protracted and it remained unclear whether the jihadists were now in full control.

Nevertheless, the group's advances even under the threat of air attacks underscore its potency and struck renewed terror into the Kurds, who fear the dam may be used as a weapon of sabotage, flooding the area and draining a precious natural resource in the parched north.

Kurdish officials have vowed to take in all new arrivals, despite the shortage of accommodation and food and water. Roads across the north have been teeming this week with cars carrying fleeing refugees from central and northern Iraq. Many are members of minority communities not yet reached by Isis, who are abandoning their homes because of the perceived threat.

"I'm not staying in my home for one night longer," said a Turkman passing through Irbil as he fled from nearby Kirkuk. "The sense of helplessness is overwhelming. There is no one coming to get us out of this. The Kurds will defend their territory. But they won't defend the Arab territories, and neither will the Arabs. So we may as well be here."

Food and water was dropped to members of the Yazidi community stranded on Mount Sinjar, not far from their ancestral home, alleviating for now a catastrophic humanitarian crisis. But the mountain remains encircled by jihadists who have vowed to kill any Yazidis who descend.

The US dropped 24,000 litres of water and 5,300 ready-to-eat meals early on Friday. Turkey has also airlifted supplies, along with Iraqi forces. Peshmerga troops are understood to be trying to break the siege of Mount Sinjar and they helped get an overland supply line to an edge of the mountain on Thursday. However they have not returned to the positions they abandoned after the Isis advance this week, which may have changed the geo-ethnic make-up of northern Iraq for good.

Save the Children described the speed of the displacement caused by Isis as unprecedented, adding that thousands of families had entered the Kurdistan region of Iraq from Qaraqosh in the last 48 hours. That exodus came days after almost 200,000 people fled Sinjar. According to the charity, 1.2 million Iraqis have been displaced in the past two months since the fighting began, placing the humanitarian relief effort under huge pressure.

"I have never known such a rapid moment of displacement," said its Iraq country director, Tina Yu. "We're seeing children and families who have fled their homes, often in the middle of the night, fearing for their lives and with nothing but the clothes on their backs.

"When they find somewhere safe to shelter they often don't have the means to buy basic necessities like food and medicine, and they don't know if their lives will ever be the same again."

A Christian woman, Ruba Khanfar, said her community, in Qaraqosh, had been shattered past 48 hours. "We couldn't believe how quickly they moved," she said in reference to the Isis militants who advanced unopposed.

"Even with this mess in the country, we still had faith that someone would help us, that it would stay together. It's all gone now. Iraq is finished."