China killed or imprisoned as many as 20 US intelligence sources from 2010 to 2012 as a network of spies that had taken years to build was unwound, The New York Times reported on Saturday.

The newspaper described what it called a massive intelligence breach responsible for impeding US spying operations in the communist state for several years.

CIA mole?

The Times said investigators are split over whether a mole within the CIA betrayed the sources, or whether the Chinese hacked the intelligence agency's covert communications system. Others think the breach could have been the result of careless spy work.

The newspaper cited 10 current and former American officials, who spoke on condition of anonymity and described how Beijing systematically dismantled the CIA's spying efforts.

They said the breach was a severe setback for the US intelligence network that had been working at its highest level in years. Almost every employee of the US embassy in Beijing was investigated at one point, the Times reported.

At least a dozen CIA sources killed

The CIA had been receiving high-quality information about the Chinese government until 2010, when the data began to dry up.

The CIA sources began disappearing in early 2011, the paper said.

The Times said at least a dozen CIA sources were killed, including one who was shot in front of colleagues in a clear warning to anyone else who might be spying. Several others were jailed.

The investigation ultimately centered on a former CIA operative who worked in a division overseeing China, the newspaper said, but there was not enough evidence to arrest him. He is now living in another Asian country and has been questioned.

The breach was considered particularly damaging, with the number of assets lost rivaling those in the Soviet Union and Russia who perished after information was passed to Moscow by spies Aldrich Ames and Robert Hanssen, the report said.

Ames was active as a spy in the 1980s and Hanssen from 1979 to 2001.

By 2013, the FBI and CIA concluded that China no longer had the ability to identify American agents, the Times said.

US intelligence agencies have since been trying to rebuild their spy network in the country.

mm/cmk (AFP, AP, Reuters)