Two time bombs exploded early Tuesday at building sites for new police stations in Athens and the northern Greek city of Thessaloniki, police said. Neither blast caused any injuries.
Authorities said the anti-terrorist squad was examining the blasts, both caused by devices consisting of timing mechanisms and large gas canisters. The Athens explosion was preceded by an anonymous warning call to local media, and authorities had cordoned off the area.
A Greek television station said its morning news program received a call claiming responsibility for both of Tuesday's attacks on behalf of a group calling itself Conspiracy Nuclei of Fire: Athens and Thessaloniki. Police were examining the authenticity of the call.
The group has often carried out arson attacks in the past. Although some of its previous attacks have been coordinated, the explosive devices used have usually consisted of small camping gas canisters and have not been known to use timing mechanisms.
Greece has seen an increase in bombings, arson attacks and shootings in recent months, after riots in December sparked by the fatal police shooting of a teenager. Most have targeted the police, banks and the media and have been attributed to shadowy far-left or anarchist groups.
None of the bombings has caused injuries, but a 21-year-old riot policeman was critically wounded in a January shooting. That attack was claimed by a group called Revolutionary Struggle, best known until then for firing a rocket-propelled grenade into the U.S. Embassy building in 2007.
U.S. terror designation
On Monday, U.S. State Department spokesman Ian Kelly said Secretary of State Hillary Rodham Clinton had designated Revolutionary Struggle as a foreign terrorist organization under the U.S. Immigration and Nationality Act. The designation bars any form of support from people or organizations within the United States.
Greece suffered 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.
The State Department has maintained its designation of a terrorist group for November 17, although Greek authorities say the group has been dismantled.
Clinton's April 22 findings, which followed five years of review within the State Department, lifted the terrorist organization designation from another Greek group, Revolutionary Nuclei.