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Iran could contemplate cooperating with its old adversary the United States on restoring security to Iraq if it saw Washington confronting "terrorist groups in Iraq and elsewhere", Iranian President Hassan Rouhani said on Saturday.

Rouhani, a relative moderate who has presided over a thaw in Iran's relations with the West, added that Tehran stood ready to provide help to Iraq within the framework of international law, but so far Baghdad had not requested such assistance.

Hundreds of young Iraqi men gripped by religious and nationalistic fervor streamed into volunteer centers Saturday across Baghdad, answering a call by the country's top Shiite cleric to join the fight against Sunni militants advancing in the north.

Dozens climbed into the back of army trucks, chanting Shiite slogans and hoisting assault rifles, pledging to battle the Sunni group known as the Islamic State of Iraq and the Levant, which has launched a lightning advance across the country.

"By God's will, we will be victorious." said one volunteer, Ali Saleh Aziz. "We will not be stopped by the ISIL or any other terrorists."

The massive response to the call by the Iranian-born Grand Ayatollah Ali al-Sistani, issued via his representative Friday, comes as sectarian tensions are threatening to push the country back toward civil war in the worst crisis since US forces withdrew at the end of 2011.

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