A little known group claimed responsibility Monday for a bomb planted outside an Athens (search) branch office of a U.S. bank.

Greek police used a controlled explosion late Sunday to destroy the small time bomb, which was hidden in a backpack and placed outside a Citibank branch in the northeastern Athens suburb of Halandri. There were no injuries or damage.

The Greek group calling itself Revolutionary Struggle claimed responsibility in a telephone call to the Athens newspaper Elftherotypia. Police said the bomb was made out of two sticks of dynamite, a detonator and a timing device.

The group did not give a reason for the bomb, which raised concern among security officials preparing for the Aug. 13-29 Olympics (search). The bank is located on a main road leading to the Olympic complex, about two miles away.

On Sept. 5, the same group claimed responsibility for two powerful bomb blasts at Athens' main court complex. The blasts, which wounded a police officer and damaged a building in the closely guarded court compound, were linked to the trial of members of Greece's deadliest terrorist group, known as November 17.

Security in Greece has been increased following last week's deadly train attacks in Madrid. A government task force on terrorism met Monday to discuss the implications of the Madrid attacks on Athens' preparations for the Olympics — which have a record security budget of more than $800 million.

Greece's defense minister also discussed security issues with the U.S. ambassador to Greece, Thomas Miller, on Monday.

Anti-Olympics groups have in recent weeks carried out a number of firebomb attacks to protest the games being held in Athens.

A number of fringe groups, mostly made up of self-proclaimed anarchists and arson gangs, have in the past targeted various businesses and diplomatic vehicles. Such groups have in recent years carried out hundreds of arson attacks and firebombings, mostly going after automatic teller machines at banks.