Three to four mortar rounds landed in the so-called "green zone" - the area housing the main US compound - Aljazeera's correspondent reported on Tuesday.
There have been no reports of casualties following the attack, which took place between 9:00pm and 9:30pm (19:00 and 19:30 GMT). Plumes of smoke were seen rising from the compound.
Eyewitnesses told Aljazeera the mortar fire appeared to come from the al-Bayya or Dora districts of Baghdad.
US forces have come under increasingly bold attack in the city, which has seen several resistance strikes over the past week.
Insurgents fired at least three mortars or rockets at the occupation headquarters itself last Tuesday, leaving four US soldiers wounded.
Earlier this Tuesday, two bombs exploded in the southern city of Basra, where Iraqi police said four Iraqi civilians were killed. Nine people, including schoolchildren, were hurt in one blast, according to police colonel Muhammad Khaziam al-Ali.
A second blast echoed over downtown Basra about midday (09:00 GMT), an AFP correspondent reported, but there were no reports of further casualties.
Also on Tuesday, a bomb exploded outside a court in Baghdad when US soldiers were bringing prisoners out of the building. Six people were injured, among them two policemen.