At least one rocket or mortar shell was fired on Wednesday into the Green Zone in Iraq's capital, eyewitnesses said.
Black smoke could be see rising after a blast from the heavily fortified area. Loudspeakers warned residents to "take cover and await further instructions."
There was no immediate word on casualties or damage, but helicopters could be seen flying into the area soon after the explosion.
The area has been repeatedly targeted by militants in the past two weeks. Two US soldiers died in a similar attack last week and a number were wounded.
The Green Zone houses the US and British embassies and much of the Iraqi government. (AP)