An explosion has hit the US embassy compound in the centre of the Greek capital, Athens.
| The US embassy is a focal point for dissent in Athens|
Police said a small rocket-propelled grenade or similar device was fired into the front of the embassy.
Fire engines and police cordoned off the area around the fortified building. The US State Department said there were no injuries.
The BBC correspondent in Athens says the US embassy is often the target of small-scale attacks by radical groups.
Many of the attacks are carried out by small groups following in the footsteps of the disbanded November 17 leftist group, says the BBC's Malcolm Brabant.
Emergency services rushed to the embassy, which is on one of Athens' main boulevards, after reports of an explosion shortly before 0600 (0400 GMT).
What appeared to be a rocket propelled grenade was fired at the front of the building, just above the US emblem.
According to eyewitnesses, it penetrated the building and caused a small fire.
Television pictures showed a mass of emergency vehicles and stationary traffic outside the embassy as the area was sealed off.
Greek police said a shell was fired from street level through the front of the building and landed in a toilet.
"This is an act of terrorism. We don't know where from," Attica Police Chief Asimakis Golfis told the Associated Press.
The blast was powerful enough to break windows in nearby buildings, reports said.