The powerful chairman of the Senate Armed Services Committee is ruling out airstrikes in Iraq unless the country's religious and ethnic groups join together in a formal request for US military support.

The comments from Sen. Carl Levin, D-Mich., come as the Shiite government of Prime Minister Nouri al-Maliki has asked the United States to bomb the Sunni militants of the Islamic State of Iraq and al-Sham (ISIS) as they make their way south toward the capital, Baghdad. Some Republicans are urging the administration to act now, but the White House and many lawmakers of both parties are worried about getting sucked into a sectarian conflict if Maliki does not mend fences with Sunnis and other groups.

"In particular, and this is really my key point, we should only act if leaders of all elements of Iraqi society — Shia, Sunni, Kurds and religious minorities, and also leaders of some of the clans and some of the tribes — only if all of them act in this way together," Levin told reporters on Capitol Hill. "And that is, that they join in a formal request for additional military support."

Levin described the Sunni ISIS militants as a "vicious enemy, a common enemy" that is a "threat to all of the groups in Iraq." If Iraq cannot unite against them, he said, the country is "likely" to end up splintering into sectarian enclaves.