The EU announced an urgent investigation on Wednesday after hackers with possible links to China accessed thousands of sensitive diplomatic cables.
In the latest embarrassing data breach to hit a major international organisation, the New York Times reported that hackers using similar techniques to the Chinese military gained entry to EU communications.
The cables from European Union diplomatic missions around the world reveal anxiety about how to handle US President Donald Trump as well as concerns about the behaviour of China, Russia and Iran.
The leak, discovered by cybersecurity firm Area 1, recalls the publication by Wikileaks of a vast haul of US State Department cables in 2010, though in the EU case the trove is much smaller and consists of less secret communications, the NYT reported.
EU officials said they have begun a probe into the leak, which comes with Europe on high alert for malign online activity in the run-up to key parliamentary elections in May next year.
"The Council Secretariat is aware of allegations regarding a potential leak of sensitive information and is actively investigating the issue," the EU Council, which represents member states, said.
"The Council Secretariat does not comment on allegations nor on matters relating to operational security. The Council Secretariat takes the security of its facilities, including its IT systems, extremely seriously."
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