Hackers allegedly linked to China were able to access thousands of sensitive EU diplomatic cables for more than three years, according to a New York Times report.

The dispatches from EU missions around the world reveal the bloc's struggles to deal with US President Donald Trump, as well as anxieties over China, Iran and Russia.

In one of the cables, EU officials in Moscow describe a July 2018 meeting between Trump and Russian President Vladimir Putin in Helsinki as "successful (at least for Putin)."

Another detailed report of a working meeting and dinner between EU officials and Chinese leader Xi Jinping quoted President Xi as saying the US was "behaving as if it was fighting in a no-rules freestyle boxing match" over its trade policy.

The embarrassing document leak comes as a senior intelligence official, quoted in the Times report, revealed America had "repeatedly" warned the EU its communications systems were vulnerable to hackers.

'Messaging efforts' to deal with Trump

The hacked cables reveal further Chinese and EU frustration with President Trump.

EU officials quoted President Xi as accusing the US of "bullying" China in a 'no-rules freestyle boxing match'

President Xi is also quoted as saying his country "would not submit to bullying" from the US and would take countermeasures on tariffs "even if a trade war hurt everyone."

Read more: What will keep China and Russia from building a new world order?

Meanwhile, Caroline Vincini, the deputy head at the EU's Washington mission, advised diplomats to describe the US as "our most important partner" to combat the Trump administration's negative view of the bloc, even though they disagreed on issues such as the Iran Nuclear Deal.

Further cables detail the situation in Ukraine, where government forces continue to battle pro-Russian separatists. They include a warning that Moscow may have deployed nuclear warheads in the "hot-zone" of Crimea, which Russia seized from Ukraine in 2014.

EU officials called Trump's meeting with Putin in Helsinki a success for the Russian leader

'No doubt' hack was linked to Chinese

The hackers apparently got into the diplomatic communications by targeting EU officials in Cyprus with a run-of-the-mill phishing campaign.

Security firm Area 1 discovered the attack and passed along 1,100 of the leaked cables, the newspaper said.

The company said the techniques used were similar to those employed by an elite Chinese military unit.

"After over a decade of experience countering Chinese cyberoperations and extensive technical analysis, there is no doubt this campaign is connected to the Chinese government," Area 1 expert Blake Darche was quoted as saying.

The EU is attempting to overhaul vulnerable communication channels, and says more secretive information is handled differently on more secure systems.

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