Three or four loud explosions have rocked central Baghdad in or near the compound housing the U.S.-led coalition authority early Friday.

Just after midnight local time, three or four small booms could clearly be heard throughout Baghdad, followed by several larger explosions.

A plume of smoke could be seen rising over the so-called "Green Zone," which is the section of downtown housing the U.S.-led coalition authority. A loud siren rang out across the city, appearing to come from the Green Zone.

U.S. military spokespeople could not say what caused the blasts or even whether they had actually occurred inside the coalition compound. But the series of explosions in rapid succession sounded to experienced witnesses like a mortar attack.

The extent of the damage has not been determined, and it is not clear whether there are any casualties.

It has been some time since the last direct attack on the coalition headquarters, but several suicide bombings have rocked U.S. military facilities over the last two days, and a military cargo plane was forced to make an emergency landing at the Baghdad airport earlier this week after being hit by a missile fired from the ground.