A bomb exploded outside a Citibank branch in Athens early Monday causing damage but no injury.

A police statement said the bomb went off at 3 a.m. local time (0100 GMT) Monday in the Nea Ionia district of the capital. The device had been planted behind the two-story bank building, which suffered moderate damage.

There was no claim of responsibility for the attack, and police said they had received no warning call.

Police said the bomb was detonated from very close to the blast site, with the use of electric cables. A police spokeswoman said the attackers used "a medium-sized improvised device," and the damage to the building was not severe. She was speaking on customary condition of anonymity.

The target of the attack pointed to Greek far-left militant groups, which have become increasingly aggressive following the police shooting of a 15-year-old boy in December — an incident that sparked the country's worst riots in decades.

Nobody has been killed so far, but authorities are alarmed that the terror tactics appear to demonstrate a desire to carry out indiscriminate slaughter.

Last month, police destroyed a car bomb abandoned outside Citibank offices in Athens, which contained enough explosives to crumble a four-story building. There has been no claim of responsibility.

The Feb. 18 failed bombing followed two gunfire attacks on a private Greek television station and a police station, which caused no injury. A new far-left group called Sect of Revolutionaries claimed responsibility for those two strikes.

In January, a far-left group called Revolutionary Struggle claimed responsibility for a Jan. 5 shooting that seriously wounded a 21-year-old riot policeman in central Athens.

The group is best known for firing a rocket-propelled grenade into the U.S. Embassy in Athens in 2007.

Greece has faced targeted attacks by domestic terrorist groups for decades. But authorities believed the problem had diminished after the arrest of several members of the country's deadliest group, November 17, following a botched bombing in 2002.