The US Embassy is seen after an explosion in Athens early Friday, January 12, 2007. [AP]

ATHENS, Greece - Assailants fired a rocket at the the U.S. embassy in Athens on Friday but no one was hurt in the attack, which anonymous callers said was staged by Greek leftists, authorities said.

In the most serious attack against the mission in 10 years, the small rocket launched from across the street shattered windows and woke up nearby residents in the central Athens area at 5:58 AM (0358 GMT).

"There is one or two anonymous phone calls which claim that the Revolutionary Struggle was behind the attack," Public Order Minister Byron Polydoras told reporters outside the embassy.

The leftist guerrilla group has emerged as the most serious domestic threat since the dismantling of the deadly November 17 group in 2002. It claimed an assassination attempt on Greece's Culture Minister last May and a bomb at the Economy Ministry which wounded two people and damaged buildings 13 months ago.

"I am treating this as a very serious attack," U.S. ambassador to Athens, Charles Ries, told reporters outside the mission. "The embassy was attacked in a senseless act of violence. There were no injuries."

Greek police said the rocket was launched from a building across the road from the mission, which is often the target of Greek protests and demonstrations, and landed inside a toilet on the third floor.

"The projectile caused slight damage to the facade's glass and the ceiling," police said in a statement.

ATTACK CONDEMNED

Greek Foreign Minister Dora Bakoyanni rushed to the embassy to meet Ries and condemn the attack.

"Such acts have cost us dearly in the past," she told reporters. "The government will do everything in its power so they are not repeated."

In February 1996, November 17 claimed responsibility for a rocket attack at the back of the compound, which caused minor damage to three diplomatic vehicles and some surrounding buildings.

Once Greece's biggest security threat, the group was dismantled two years before the Athens 2004 Olympics. It had staged dozens of bombings, shootings and rocket attacks, and killed U.S. and other foreign diplomats in Greece.

Dozens of police cars surrounded the embassy and hundred of police cordoned off all roads in the area, including a major boulevard in front of the mission. Police helicopters hovered overhead.

Local residents called in to state television saying they had felt the explosion, which shattered some windows.

"It was a huge explosion, the ground shook. I woke up and rushed to the balcony to see what happened," a local resident, who was not identified, told Greek state TV.

The heavily-guarded embassy building, surrounded by a 3-metre (9-feet)-high steel fence, has guards are posted at every entrance and at street corners around it.

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