Updated

The U.S. Embassy in Athens was attacked early Friday by a rocket that exploded inside the modern glass-fronted building but caused no casualties in an attack police said was probably carried out by a domestic terror group.

Narrowly missing the embassy seal, the small 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.

Greece's Public Order Minister said the pre-dawn attack was probably carried out by a domestic terrorist group — raising fears of resurgent violence by far-left Greek militants.

Greek officials condemned the attack and said they would move quickly to discover who was responsible for firing the rocket.

"There were no injuries and very minimal damage," U.S. Ambassador Charles Ries told reporters outside the embassy. The rocket struck near his office.

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The rocket, which police said was a 2.36-inch weapon probably fired from a Russian-made launcher, struck a large marble beam on the third floor of the embassy, just above and to the left of the seal. It then shattered a window and landed near some toilets.

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

He said Greece "strongly condemns" the attack on the heavily guarded three-storey building — the first major attack against a U.S. target in Greece in more than a decade.

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

Polydoras said police were examining the authenticity of phone calls to a private security company claiming responsibility on behalf of a militant left-wing group.

"There are one or two telephone calls, from unknown callers, who claimed that the Revolutionary Struggle assumes responsibility," Polydoras said. "We cannot rule out that they were genuine."

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

Police cordoned off streets around the building after the explosion shortly before 6 a.m (0400 GMT).

Investigators were examining video tapes from traffic cameras and embassy surveillance video and were canvassing the neighborhood around the embassy, located near central Athens next to the capital's main concert hall.

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It was unclear if the launcher had been found, but the rocket was fired from across a six-lane boulevard.

"At 5:58 a.m. an explosion was caused by a self-propelling shell, causing minor damage at the front of the building," a police statement said.

Ries said there had been no warning of the attack, which he said came at a time when "the embassy had very few people in it."

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

Greek Foreign Minister Dora Bakoyannis visited the embassy after the blast.

"I came here to express the solidarity of the Greek people following this deplorable action," she said.

"Such actions in the past have had a very heavy cost for the country. ... The Greek government is determined to undertake every effort to not allow such phenomena to be repeated in the future."

A U.S. State Department spokesman confirmed that the attack caused no injuries.

"There are no injuries or casualties of any kind," Kurtis Cooper said. "Police have responded. As a result the embassy will be closed today, Friday."

Polydoras, the Public Order Minister, said police would set up a special task force on the attack, headed by a former counterterrorism chief who eradicated the far-left 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.

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

November 17 carried out a similar rocket attack against the U.S. embassy in 1996, causing minor damage and no injuries.

Several obscure militant groups have appeared since the November 17 members were arrested.

Radical groups Revolutionary Struggle and Popular Revolutionary Action were blamed for the bombings of three government ministries in 2005.