The United States said it could launch air strikes and act jointly with its arch-enemy Iran to support the Iraqi government, after a rampage by Sunni Islamist insurgents across Iraq that has torn up traditional alliances in the Middle East.
Militants from the Islamic State of Iraq and the Levant 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.
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Joint action between the United States and Iran to help prop up the government of their mutual ally would be unprecedented since Iran's 1979 revolution, demonstrating the urgency of the alarm raised by the lightning insurgent advance.
U.S. Secretary of State John Kerry called the advance an "existential threat" for Iraq. Asked if the United States could cooperate with Iran against the insurgents, Kerry told Yahoo News: "I wouldn't rule out anything that would be constructive."
As for air strikes: "They're not the whole answer, but they may well be one of the options that are important," he said. "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."
Britain, once Washington's only major battlefield ally in Iraq, announced it had already reached out to Iran in recent days. A U.S. official said meetings with Iran could come this week on the sidelines of separate international nuclear talks.
Iran has longstanding ties to Iraq's Prime Minister Nuri al-Maliki and other Shi'ite politicians who came to power in Iraq after the U.S. invasion that toppled Saddam Hussein in 2003.
ISIL seeks a caliphate ruled on mediaeval Sunni Muslim precepts in Iraq and Syria, fighting against both Iraq's Maliki and Syria's Bashar al-Assad. It considers all Shi'ites to be heretics deserving death and has boasted of massacring hundreds of Iraqi troops who surrendered to its forces last week.
Its fighters are joined by other armed Sunni groups, who oppose what they say is oppression by Maliki's Shi'ite-led government in Baghdad.
ISIL fighters and allied Sunni tribesmen overran yet another town on Monday, Saqlawiya west of Baghdad, where they captured six Humvees and two tanks, adding to an arsenal of U.S.-provided armor 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.
"It was a crazy battle and dozens were killed from both sides. It is impossible to reach the town and evacuate the bodies," said a medical source at a hospital in the nearby largely insurgent-held city of Fallujah.
Overnight the 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.
"The city was overrun by militants. Severe fighting took place, and many people were killed. Shi'ite families have fled to the west and Sunni families have fled to the east," said a city official who asked not to be identified.
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. 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.