A Greek urban guerilla group claimed on Tuesday responsibility for an armed attack against the German ambassador's residence in Athens in December which caused no injuries. In a proclamation sent to a Greek daily, the so-called Popular Rebels Group said that the December 30 attack came in "the memory" of a pensioner who committed suicide in front of the Greek parliament in April 2012 in protest at austerity. The group which has also claimed responsibility for a similar attack against the conservative ruling party's headquarters in January 2013 calls for new attacks in particular during the six months of the Greek EU Presidency which ends in July this year. Greece has suffered from domestic violent attacks for decades. Attacks against political, police and financial targets have increased since the start of the financial crisis in 2009. Over the past month Greek authorities are on alert for fear of a new serious hit, following the escape of Christodoulos Xiros in early January during a leave from prison. Xiros, member of the now dismantled "November 17" group which was active until 2003, threatened with "continuous armed struggle" in a proclamation released three weeks ago. Tuesday's new warning came as police officers in Athens expressed concern over the disappearance of another suspect of participation in an urban guerilla group who failed to appear in a police station this week under the terms of his conditional release pending trial. Constantinos Sakkas is charged with participation in the Conspiracy of the Nuclei of Fire group, which has been linked to a series of bomb attacks in recent years.