Iraq's Prime Minister Nouri Maliki has told the BBC that he hopes jets from Russia and Belarus will turn the tide against rebels in the coming days.
"God willing within one week this force will be effective and will destroy the terrorists' dens," he said.
He said that the process of buying US jets had been "long-winded" and that the militants' advance could have been avoided if air cover had been in place.
Isis and its Sunni Muslim allies seized large parts of Iraq this month.
Mr Maliki was speaking to the BBC's Arabic service in his first interview for an international broadcaster since Isis - the Islamic State in Iraq and the Levant - began its major offensive.
"I'll be frank and say that we were deluded when we signed the contract [with the US]," Mr Maliki said.
"We should have sought to buy other jet fighters like British, French and Russian to secure the air cover for our forces; if we had air cover we would have averted what had happened," he went on.
He said Iraq was acquiring second-hand jet fighters from Russia and Belarus "that should arrive in Iraq in two or three days".
The government has struggled to hold back the militants' advance from the north and west.
The US, which backs the Iraqi government, has stressed that the militants can only be defeated by Iraq's own forces.
Iraq has also been receiving support from Iran, with whom its Shia Muslim leaders have close links.
Mr Maliki also confirmed that Syrian forces had carried out air strikes against Islamist militants at a border crossing between Iraq and Syria.
He said Iraq had not requested the strikes but that it "welcomed" them.
"They carry out their strikes and we carry out ours and the final winners are our two countries," he said.
Military and rebel sources say the strike took place inside Iraq, at the Qaim crossing, although Mr Maliki said it was carried out on the Syrian side.
Militant sources have been reporting for two days that Syrian jets hit the Iraqi side of Qaim, and also Rutba which is further inside Iraq.
The militants say 70 people were killed in the first attack and 20 in the second.
Fighting has been reported on Thursday, with Iraqi special forces flying into the university in the city of Tikrit and clashes ensuing.
Also on Thursday, the charity War Child condemned the "gravest violations against children's rights" in the course of the conflict, including "recruitment and use of children by armed groups".
The charity also says it fears the "risk of sexual violence for children is extremely high".
Analysis: Jim Muir, BBC News, Irbil, northern Iraq
So far, the Iraqi army has been unable to launch a strategic counter-offensive to drive the rebels back.
The addition of three Iranian-backed Shia militias to its forces in the field has added to the perception that this is a Shia army fighting to impose Shia rule on Sunni areas.
Its chances of re-conquering the lost ground appear very slight. And if it did, it would be crushing and further displacing Sunni populations to plant the state flag on the smoking ruins.
It's now taken for granted by most Iraqi politicians that the Sunnis have carved out their own area, and that things will never be the same.
Mr Maliki is trying to form a new government but has rejected calls to create an emergency coalition which would include all religious and ethnic groups.
He said on Wednesday that forming a broad emergency government would go against the results of April's parliamentary elections, which were won by his alliance of Shia parties.
His political rival, Ayad Allawi, had proposed forming a national salvation government.
Meanwhile, Masoud Barzani, the leader of Iraqi Kurdistan visited the northern city of Kirkuk for the first time since it was seized by Kurdish forces earlier this month.
It fell into the hands of Kurdish fighters when Iraqi troops fled in the face of the Islamist advance.
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