***Updated****

Fox News just broke in at the top of the hour to report that according to Greece police, there was an explosion inside the US Embassy in Greece.

Here's the report from Reuters:

ATHENS (Reuters) - 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.

CNN reports that no one was injured and that the blast took place just before 6am.

According to this AP report, the explosion took place inside the main embassy building.

The Israel National News reports that the blast was heard on the third floor.

Update: Sky News reports that Greece police are saying that it was an attack on the US embassy:

An explosion at the US embassy in Athens was caused by a rocket or bomb, police in Greece say.

Witnesses described a loud explosion in the main embassy building.

Police in Greece said the explosion was caused by an "attack".

"The blast was caused by an attack from the outside of the embassy. We are not in a position to say what kind of device was used," a senior police official said.

Update II: This blast is now being called terrorism.

ATHENS, Greece - A rocket was fired at the U.S. embassy early Friday, striking the front of the building but causing no injuries. A senior police official said the blast was an act of terrorism.

Police cordoned off streets around the heavily guarded building after the explosion shortly before 6 a.m. The shell struck the third floor and smashed glass in nearby buildings.

Investigators found the device used to fire the rocket shell at a construction site near the embassy.

"This is an act of terrorism. We don't know where from," Attica Police Chief Asimakis Golfis said. "There was a shell that exploded in the toilets of the building ... It was fired from street level."

It landed in a toilet - I can't think of a better place for the terrorists' rocket to land. Well, actually I can, but let's just leave at that.

Stop the ACLU has a round up.