Artillery blasts rained down early Sunday morning on Baghdad's Green Zone, a heavily fortified area of the capital that houses the Iraqi government, major embassies and thousands of U.S. troops.

The attack came in three waves of rocket and mortar fire and lasted about 15 minutes. Black smoke could be seen billowing up from buildings in one part of the Green Zone.

It was not immediately known what buildings had been struck or whether there were any casualties.

Sirens blared, warning people to take cover, and a U.S.-installed public-address system cautioned to stay away from windows during the barrage.

The Green Zone, which covers about 10 square kilometres on the left bank of the Tigris River, has been a frequent target of munitions attacks. The U.S. military has blamed several past Green Zone bombardments on rogue Shia paramilitaries.