An anti-tank rocket was fired into the US embassy in Athens in the early hours of this morning, causing damage to the building’s interior, and raising fears of a resurgence in domestic terrorism.

No injuries were caused by the eastern-European-made device, which penetrated the building near its front entrance at around 6am (4am GMT) and exploded inside, damaging a toilet on the third floor, which also houses the ambassador’s office.

But the attack, which shattered windows and awoke residents in the area, was described as very serious by Greek and US officials, who vowed to track down the perpetrators whom they accused of seeking to fracture diplomatic ties.

"We believe it is a symbolic act," said Vyron Polydoras, Greece’s Public Order Minister. "It is an attempt to disrupt our country’s international relations."

A group calling itself the Revolutionary Struggle telephoned a private security company, declaring responsibility for the incident, a claim which police are now investigating.

The leftist group has emerged as the most serious domestic threat to the country since the dismantling of the deadly November 17 group in 2002, with which some analysts believe it has links. It has previously claimed responsibility for an assassination attempt against the culture minister in May and a bomb at the Economy Ministry, which injured two and damaged buildings more than a year ago.

US-owned banks and companies have been frequently targeted by small bomb attacks by dissident groups in Greece. But Friday’s incident is the most serious since the break-up of November 17, which is said to be responsible for several murders, including the assassination of the CIA station chief in Athens.

The group has been blamed for hundreds of shooting, bombings and rockets attacks, targeting US, British and other diplomats as well as military personnel. A decade ago, November 17 fired a rocket at the embassy courtyard in a night-time attack that caused no casualties.

"This is a very serious attack," US ambassador Charles Ries told reporters outside the embassy, a modern glass front building which is one of the best-guarded in the country, as hundreds of officers blocked off roads in the vicinity and police helicopters hovered overhead.

"It was a huge explosion, the ground shook. I woke up and rushed to the balcony to see what happened," a local resident told Greek TV. "The explosion sounded in the courtyard, then a fire broke out over the central entrance," another witness told a radio station.

A police official said that the rocket is thought to have fired from a neighbouring construction site, where a building was recently demolished. "We have not yet found the device that launched the rocket," he added.

In Washington, the State Department confirmed the explosion. "There are no injuries or casualties of any kind. Police have responded. As a result, the embassy will be closed today Friday the 12th of January," State Department spokesman Kurtis Cooper said.

The police cordon around the embassy, located near the Athens city centre, caused massive traffic gridlock amid rush-hour traffic. Police were only allowed access to the building at least 15 minutes after the blast as US embassy officials sought to determine what had happened, a police source said.

Police had stepped up surveillance of militant groups in recent years, particularly in the run-up to the Athens 2004 Olympics. "Incidents such as these have cost (Greece) a great deal politically, economically and in terms of prestige," Greek Foreign Minister Dora Bakoyannis said after visiting the embassy.

"The Greek government is determined to make every effort, as it has achieved in the past, so that such incidents are not permitted to occur again," she said.

Greek police were working together with American experts on the investigation, officials said.

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