A militant anarchist group has claimed responsibility for a bombing at a package shipping firm in Greece, and is threatening further attacks on judges, police and prosecution witnesses in a terrorism trial.

No one was hurt in Sunday's blast in Athens, which damaged the closed Swift Mail offices.

In 2010, a Greek anarchist group used the firm's offices to send parcel bombs that targeted Athens embassies and European officials, including one that reached German Chancellor Angela Merkel's office in Berlin.

In a posting Friday on a left-wing forum, another anarchist group, the Informal Anarchist Federation, said it attacked Swift Mail because a worker's testimony had helped police arrest anarchist suspects now on trial in Athens.

The group said it would target witnesses, judges and police "with bullets, bombs and knuckledusters."