Attackers have fired a rocket at the US embassy compound in the centre of the Greek capital, Athens.
Fire engines and police cars sealed off the embassy
The rocket, fired from street level into the front of the embassy, caused minor damage to the building but no-one was injured.
The US envoy condemned the "very serious attack", as fire engines and police cordoned off the area.
Greece's Public Order Minister Vyron Polydoras said it was "very likely" a domestic group was behind the attack.
He said police were investigating claims that a left-wing radical group, Revolutionary Struggle, was responsible.
The blast was reported in the early morning at the embassy, which is on one of the main boulevards in Athens.
Police said a rocket was fired at the US eagle emblem at the front of the building.
"A self-propelled explosive was fired at 0558 (0358 GMT) from the surrounding area, causing minor damage to the front windows and the roof," a statement from the Greek public order ministry said.
The rocket landed in a toilet on the third floor of the building, which also houses Ambassador Charles Ries's office.
"I am treating this as a very serious attack," Mr Ries said. "The embassy was attacked in a senseless act of violence."
Mr Ries said the reliability of any claims of responsibility would have to be assessed.
Television pictures showed a mass of emergency vehicles and stationary traffic outside the embassy as the area was sealed off.
The US embassy is one of the most fortified and tightly guarded buildings in the region and is frequently the target of protests in a country brimming with potent anti-Americanism, says the BBC's Malcolm Brabant in Athens.
In February 1996, it suffered minor damage when unknown attackers - thought to be leftist radicals - fired a rocket at it.
In the past, the far-left November 17 group - now disbanded and whose leaders were jailed in 2003 - attacked Greek, US and other foreign targets, killing more than 20 people.
Revolutionary Struggle - a group which emerged after November 17 disintegrated - is regarded by security experts as Greece's most active terrorist group, our correspondent says.
It has broadly the same left-wing, anarchic anti-capitalist agenda as November 17 and vigorously opposes America's intervention in Iraq and Afghanistan, as well as the economic and social policies of the current conservative Greek government.
It has carried out a number of bombings and attacks in recent years.