An explosion caused by a rocket attack shook the US embassy in central Athens early on Friday morning, but no injuries were reported, Greek police said.

Officers said the rocket, launched from a building across the street, left a large hole in the embassy almost half a meter deep.

Witnesses said the explosion was so powerful that it could be clearly heard from over 500 meters away.?

Police cordoned off the area around the heavily guarded building following the explosion shortly before 6 AM. Investigators were examining what was believed to be the device used to fire the rocket shell from 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."

Embassy officials confirmed the explosion, adding that no one had been injured. US ambassador Charles Reis said the damage was "not extensive."

"There can be no justification for such a senseless act of violence ... The embassy was occupied at the time (but) nobody was hurt," he said, adding that no warning had come of the attack.

"We're treating it as a very serious attack and we will determine what it is when we know (more)," he said.

Authorities were searching apartment buildings near the US Embassy and a nearby hospital for evidence.

"The Greek Police are working closely with the embassy authorities," a police statement said.

Traffic froze to a standstill across parts of central Athens, as police and emergency services scrambled to the embassy building, a frequent destination for protest groups.

It was the first major attack against a US target in over a decade in the South European country, following the dismantling of Greece's far-left November 17 terrorist group. The group was blamed for killing 23 people - including US, British and Turkish officials - and dozens of bomb attacks and was shut down in 2002.?

In 2003, a special court gave multiple life sentences to November 17's leader, chief assassin and three other members. Lesser sentences were handed to 10 others.

In February 1996, unidentified assailants fired a rocket at the US embassy compound in Athens, causing minor damage to three diplomatic vehicles and some surrounding buildings.

In November last year, Greek riot police fired tear gas towards demonstrators marching to the US embassy in Athens who chanted slogans including "Bush the butcher, out of Iraq" and "The USA is the real terrorist".

(Xinhua News Agency, Chinadaily.com via agencies January 12, 2007)