President Barack Obama considered options for military action to support Iraq's besieged government on Monday, June 16 but made no decision on the U.S. response to a Sunni militant onslaught that has threatened to tear the country apart, Reuters reported.

Obama, who discussed the crisis with his top national security advisers, has made U.S. action contingent on Iraqi Prime Minister Nuri al-Maliki's taking steps to broaden his Shi'ite-dominated government.

"The president will continue to consult with his national security team in the days to come," the White House said, without elaborating. A senior U.S. official said Obama had not yet decided on a course of action.

Militants from the Islamic State of Iraq and the Levant group 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 that oppose what they say is oppression by Maliki. The UN human rights chief said forces allied with ISIL had almost certainly committed war crimes by executing hundreds of non-combatant men in Iraq over the past five days.

U.S. and Iranian officials discussed the crisis in Vienna on the sidelines of separate negotiations about the Iranian nuclear program, the two sides each said. Both ruled out military cooperation.

A U.S. official said the talks did not include military coordination and would not make "strategic determinations" over the heads of Iraqis.

"Iran is a great country that can play a key role in restoring stability in Iraq and the region," a senior Iranian official told Reuters. But he added: "Military cooperation was not discussed and is not an option."

Any joint action between the United States and Iran to help prop up their mutual ally in Baghdad would be unprecedented since Shi'ite Iran's 1979 revolution, a sign 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 Tehran against the insurgents, Kerry told Yahoo News: "I wouldn't rule out anything that would be constructive."

As for airstrikes: "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."

Iran has long-standing ties to Maliki and other Shi'ite politicians who came to power in U.S.-backed elections.