A rocket hit the US embassy causing damage but no casualties, according to officials.

Polydoras said there had been at least two claims of responsibility to a security company, including one who said he represented Revolutionary Struggle, an extreme left-wing group which is considered the most dangerous militant group operating in Greece.

 It has carried out six attacks in recent years.

 It claimed responsibility for a May 30 explosion near the home of a former security minister George Voulgarakis.

Previous attacks by the group include one on a police station 100 days before the 2004 Athens Olympics and another in December 2005 against the economy ministry in central Athens.

Attackers fired a rocket into the US embassy in Athens early Friday, damaging the building but causing no casualties, officials said.

"This is a very serious attack," US ambassador Charles Ries told reporters outside the embassy, one of the best-guarded buildings in the country.

 Police said the rocket penetrated the building near the front-entrance emblem and exploded inside, damaging a toilet on the third floor. It was not immediately known who was responsible.

 Several windows were smashed around the emblem on the facade and there are burn marks on the adjacent wall.

 The police said it appeared the rocket was fired from the site of a demolished building opposite the embassy.

 Greek police said the explosion occurred at 5:55 am (0355 GMT).

 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 in a brief statement.

 A police cordon in around the embassy, 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.

 "The explosion sounded in the courtyard, then a fire broke out over the central entrance, where the emblem is situated," one Greek witness told private Skai radio.

 There was no immediate claim of responsibility and no indication who was behind the attack.

 A decade ago, the November 17 militant group fired a rocket at the embassy courtyard, a night-time attack that caused no casualties. The group has since been broken up.

 US-owned banks and companies have been frequently targeted by small bomb blasts in Greece.

 But this incident is the most serious since the 2002 break-up of November 17, which was blamed for several deadly attacks against foreign diplomats and military personnel, including the assassination of CIA station chief in Athens.

 Police had stepped up surveillance of militant groups in recent years, particularly in the run-up to the Athens 2004 Olympics.

 The US ambassador said the incident was under investigation and that the embassy would reopen as soon as possible.