Greece's foreign ministry on Monday said its internal email system had been penetrated by hackers and that an investigation was under way to determine the extent of the damage.
"A web posting on Monday announced a successful break-in by the 'Anonymous' group of activists into the ministry's email directory," the ministry said in a statement.
"An investigation is under way to determine the conditions and extent of the break-in, which mainly concerned material normally exchanged by email, including many press releases," it said.
Social media tweets spoke of a "huge leak" involving some 3,700 documents, including restricted material, from the foreign ministry and the Organisation for Security and Cooperation in Europe (OSCE).
A selection of the allegedly hacked material was posted online on file-sharing site Imgur. It included foreign ministry media reviews from Romania and Albania, a Greek intelligence service security review, an EU Council document on relations with Kosovo and a Council of Europe money laundering report on Albania.
The foreign ministry is tasked with organising Greece's assumption of the rotating European Union presidency in January.
The hacker group Anonymous has previously attacked other Greek state websites to protest against the austerity policies imposed on the recession-hit country for a fourth year under EU-IMF supervision.
Explore further: Digital dilemma: How will US respond to Sony hack?