Witnesses said mortars appeared to have been fired early on Sunday morning into the Green Zone from the city's Sadr City district - a stronghold of supporters of Shia leader Muqtada al-Sadr.
Sirens echoed across the zone. The US military had no immediate word on any damage or casualties.
The Green Zone has been a regular target of attack, often by mortar or car bomb, in recent months. Mortar bombs were fired at the first sessions of the interim assembly last week.