The US is contemplating talks with its arch-enemy Iran to support the Iraqi government in its battle with Sunni Islamist insurgents who routed Baghdad's army and seized the north of the country in the past week.

The US is considering air strikes to help the Iraqi government fend off the stunning onslaught by militants from the Islamic State of Iraq and Syria (ISIS) as well as possible discussions with neighboring Iran, US Secretary of State John Kerry said on Monday.

"I wouldn't rule out anything that would be constructive," Kerry told Yahoo News.

ISIS threatens to dismember Iraq and unleash all-out sectarian warfare across a crescent of the Middle East, with no regard for national borders that the fighters reject.

ISIS fighters captured the mainly ethnic Turkmen city of Tal Afar in northwestern Iraq overnight after heavy fighting on Sunday, solidifying their grip on the north.

Iraq's army is holding out in Samarra, a Tigris city that is home to a Shi'ite shrine. A convoy traveling to reinforce the troops there was ambushed late on Sunday by Sunni fighters near the town of Ishaqi. Fighting continued through Monday morning.

US President Barack Obama pulled out all American troops in 2011 and has ruled out sending them back, although he says he is weighing other military options.

Iranian President Hassan Rouhani has presided over a gradual thaw with the West, including secret talks with Washington that led to a preliminary deal last year to ease sanctions in return for curbs on Iran's nuclear program. But open cooperation against a mutual threat would be unprecedented.

Iraq is the only country closely allied to both the US and Iran, but tentative past efforts by Tehran and Washington to cooperate there were fruitless.

ISIS said it executed 1,700 soldiers out of 2,500 it had captured in Tikrit. Although those numbers appear exaggerated, the total could still be in the hundreds.

The government's collapse in the north has also allowed forces of the ethnic Kurdish autonomous region to advance, seizing the city of Kirkuk and rural areas with oil reserves.

Editorial: ISIS massacre in Iraq shames Washington