Looking for His glorious appearing! Titus 2:13

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.