Jan. 12 (Bloomberg) -- The U.S. Embassy in Athens was hit by a rocket today in what officials said may have been an attack by a Greek revolutionary group. There weren't any injuries.

The blast, at 5:58 a.m. local time, was followed by an anonymous telephone call from someone who said the Revolutionary Struggle group was responsible, Public Order Minister Vyron Polydoras said. Some damage was caused by the explosion.

U.S. Ambassador Charles Ries called the attack ``very serious'' and said there could be ``no justification for such a senseless act of violence.''

Revolutionary Struggle said it was behind a triple bomb attack in Athens in May 2004, 100 days before the city hosted the Olympic Games. Last year, the group failed in a bombing attempt that targeted a government minister and said it planned more assaults on political or business leaders. No member of the group has been arrested.

``There's been a telephone call by an unknown person claiming that Revolutionary Struggle takes responsibility,'' Polydoras said in remarks televised on Greece's state-run NET channel. He said that call, and another anonymous call, were being investigated as were the remains of the rocket.

The rocket was fired from outside the embassy compound, in central Athens on Vasilissis Sophias Avenue, the Greek Public Order Ministry said in a statement on its Web site. The area was immediately cordoned off after the blast. Traffic was permitted back onto the streets at about 9 a.m.

Since 2003

Revolutionary Struggle appeared in 2003. The U.S. State Department in 2005 included the organization on its list of ``groups of concern''.

The U.S. Embassy, one of the most heavily guarded in Europe, was similarly targeted in 1996 in an attack claimed by November 17, Greece's deadliest militant group. A series of arrests in 2002 dismantled November 17, which killed 23 people and targeted companies such as Citibank and Procter & Gamble Co. over a period of 27 years.

The crackdown stemmed criticism from the U.S. that Greece wasn't doing enough to fight terrorism. A 2001 U.S. congressional report said Greece had remained ``disturbingly passive in response to terrorist activities.'' The arrests eased concerns the group would be active during the Olympics or assist groups such as al-Qaeda.

Such acts have hurt Greece politically and economically, Foreign Minister Dora Bakoyannis said today in telephone interview. ``The Greek government is determined to take every measure, as it did in the past, to not allow such acts again.''