Militants from the Islamic State of Iraq and the Levant (ISIL) have routed Baghdad's army and seized the north of the country in the past week, threatening to dismember Iraq and unleash all-out sectarian warfare with no regard for national borders.
The fighters have been joined by other armed Sunni groups who oppose what they say is oppression by Prime Minister Nuri al Maliki, a Shi'ite.
Joint action between the US and regional Shi'ite power Iran to help prop up their mutual ally in Baghdad would be unprecedented since Iran's 1979 revolution, demonstrating the urgency of the alarm raised by the lightning insurgent advance.
US Secretary of State John Kerry called the advance an "existential threat" for Iraq. Asked if the US could cooperate with Tehran against the insurgents, Mr Kerry said: "I wouldn't rule out anything that would be constructive."
On the issue of air strikes, he said: "They're not the whole answer, but they may well be one of the options that are important. When you have people murdering, assassinating in these mass massacres, you have to stop that. And you do what you need to do if you need to try to stop it from the air or otherwise."
The Pentagon said that, while there might be discussions with Iran, there were no plans to coordinate military action with it.
A US official said meetings with Iran could come this week on the sidelines of international nuclear talks.
Iran has longstanding ties to Mr Maliki and other Shi'ite politicians who came to power in US-backed elections.
ISIL seeks a caliphate ruled on mediaeval Sunni Muslim precepts in Iraq and Syria, fighting against both Iraq's Mr Maliki and Syria's Bashar al Assad, another ally of Iran. It considers Shi'ites heretics deserving death and has boasted of massacring hundreds of Iraqi troops who surrendered to it last week.
Its uprising has been joined by tribal groups and figures from former Iraqi leader Saddam Hussein's era, who believe Mr Maliki is hostile to Sunnis.
ISIL fighters and allied Sunni tribesmen overran yet another town yesterday, Saqlawiya west of Baghdad, where they captured six Humvees and two tanks, adding to an arsenal of US-provided armour they have seized from the disintegrating army. Eyewitnesses said Iraqi army helicopters were hovering over the town to try to provide cover for retreating troops.
A medical source at a hospital in the nearby city of Falluja, largely held by insurgents since early this year, said: "It was a crazy battle and dozens were killed from both sides. It is impossible to reach the town and evacuate the bodies."
ISIL fighters also captured the mainly ethnic Turkmen city of Tal Afar in northwestern Iraq after heavy fighting on Sunday, solidifying their grip on the north.
Tal Afar is a short drive west from Mosul, the north's main city, which ISIL seized last week at the start of its push.
Fighters then swept through towns and cities on the Tigris before halting about an hour's drive north of Baghdad.
Iraq's army is holding out in Samarra, a Tigris city that is home to a Shi'ite shrine where fighting continued yesterday and an Iraqi army spokesman also reported fighting to the south of Baghdad.