At least three large explosions were heard in Baghdad tonight, and sirens wailed briefly in the Green Zone, the area housing the US-led coalition headquarters.
Rebels periodically target the Green Zone with mortars and rockets, though it was unclear if the explosions were in or near that area.
There were no immediate reports of casualties.
The explosions follow this morning's visit to Baghdad by US Secretary of State Colin Powell.
His visit came on the eve of the first anniversary of the US-led invasion that toppled Saddam Hussein.
Ten people were killed in new violence in Iraq and two Baghdad hotels were damaged in attacks yesterday.
Among the dead was an Iraqi working for Dubai-based Arab satellite television channel Al Arabiya, who was shot by US troops at a checkpoint in the city centre. His colleague was badly wounded and died later.