Officials said no one was injured in the explosion at the U.S. Embassy in Athens. ((Thanassis Stavrakis/Associated Press))

An explosion that rocked the U.S. Embassy in Athens early Friday was an "act of terrorism" caused by a rocket that was firedinto the front of the building, a senior police official said.

Callers have claimed the Greek leftist guerrilla group Revolutionary Struggle was behind the attack, Greece's Public Order Minister Byron Polydoras told reporters outside the embassy.

Police haven't confirmed whether the two anonymous phone calls are legitimate.

The rocket struck the third floorof the building but caused no injuries, the Associated Press reported.

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

The explosion shortly before 6 a.m. local time smashed glass in nearby buildings.

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

Minor damage

Police cordoned off streets around the embassy. Authorities were searching apartment buildings and a hospital nearby for evidence.

Embassy officials confirmed that an explosion had taken place and said no one had been injured. U.S. 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.

Greece's foreign minister visited the site after the blast.

"Such actions in the past have had a very heavy cost for the country— moral, financial and for the international standing of the country," said Dora Bakoyannis. "The Greek government is determined to undertake every effort to not allow such phenomena to be repeated in the future."

Traffic came to a standstill across parts of the city's downtownas police and emergency services scrambled to the embassy.

Emerged in 2003

Revolutionary Struggle, an anti-American militant group,surfaced in 2003 with a minor bombing outside an Athens courthouse. They made headlines again with a triple bombing outside an Athens police station three months before the city hosted the 2004 Olympic Games.

The group also carried out an attack last year on Culture Minister Giorgos Voulgarakis, who wasn't injured.

Friday's bombing was the first major attack againstan Americantarget in Greece in more than a decade, following the arrest of members of anotherleftistorganization, theNovember 17 militant group. The group was blamed for 23 deaths, including foreigners.

In 2003, five leaders from the group received multiple life sentences. The group carried out a minor attack against the U.S. Embassy in 1996.

With files from the Associated Press