ATHENS, Greece— The U.S. Embassy in Athens came under fire early today from a rocket that exploded inside the modern glass-front building but caused no casualties in an attack police suspect was the work of Greek leftists.

Narrowly missing the embassy emblem, the anti-tank shell pierced the building near the front entrance shortly before 6 a.m., damaging a bathroom on the third floor, which houses the ambassador's office, and shattering windows in nearby buildings.

"We're treating it as a very serious attack," U.S. Ambassador Charles Ries said.

Greece's Public Order Minister said police were examining the authenticity of anonymous phone calls to a private security company claiming responsibility on behalf of Revolutionary Struggle, a militant left-wing group.

"It is very likely that this is the work of a domestic group," Minister Vyron Polydoras said. "We believe this effort to revive terrorism is deplorable and will not succeed."

Revolutionary Struggle claimed responsibility for a May 2006 bomb attack on Culture Minister Giorgos Voulgarakis, in which nobody was hurt.

U.S.-owned banks and companies have often been targeted in small bomb attacks by groups in Greece. But today's incident was the most serious since the 2002 break-up of the far-left November 17 group, which was blamed for several attacks against foreign diplomats and military personnel, including the assassination of a CIA station chief in Athens.

Polydoras said Greece "strongly condemns" the attack.

"We believe it is a symbolic act," he said. "It is an attempt to disrupt our country's international relations."

Police cordoned off streets around the heavily guarded building after the explosion, stopping traffic in much of central Athens for more than three hours. Emergency services scrambled to the embassy building, a frequent destination for protest groups.

Investigators were examining what they believed was the device used to fire the rocket shell from a construction site near the embassy.

"This is an act of terrorism," Police Chief Asimakis Golfis said. "There was a shell that exploded in the toilets of the building. ... It was fired from street level."

Ambassador Ries said the building was not occupied at the time and the damage was minimal. The embassy is now a crime scene and will remain closed until further notice, he said.

"There can be no justification for such a senseless act of violence," said Ries, who added that there had been no warning.

Authorities were searching nearby apartment buildings and a nearby hospital for evidence.

It was the most serious attack on the mission since 1996, when November 17 carried out a rocket attack against the embassy that caused minor damage and no injuries.

Polydoras said police would set up a special task force, headed by a former counter-terrorism chief who eradicated the November 17 group in 2002. The group was blamed for killing 23 people — including U.S., British and Turkish officials — and dozens of bomb attacks.