An explosion ripped through the U.S. embassy compound in central Athens on Friday, police said.
It was not clear what caused the blast and there was no immediate word on whether there were any casualties.
"We are investigating the cause of the explosion. We are not ruling anything out," a senior police official told Reuters.
Dozens of police cars surrounded the embassy and police cordoned off all roads in the area, including a major boulevard in front of the mission. The senior police official did not say whether the explosion was set off inside the compound or whether something might have been thrown into the embassy grounds.
Police officials at the scene said that whatever caused the explosion damaged the official embassy sign outside the mission, but there was little other indication of the extent of damage inside.
The tightly guarded building is surrounded by a 3-meter (9-feet)-high steel fence. Guards are posted at every entrance and at street corners around it.
Embassy officials could not immediately be reached for comment. Local residents called in to state television saying they had felt the explosion, which shattered some windows.