John Brennan
Former CIA Director John Brennan speaks during the SALT conference in Las Vegas, Nevada, May 18, 2017. Reuters

Both the CIA and the FBI declined to comment on reports saying the Chinese government killed or imprisoned 18 to 20 CIA sources from 2010 to 2012 and dismantled the agency's spying operations in the country. It is described as one of the worst intelligence breach in decades, current and former American officials told the New York Times.

Investigators were uncertain whether the breach was a result of a double agent within the CIA who had betrayed the U.S. or whether the Chinese had hacked the communications system used by the agency to be in contact with foreign sources. The Times reported Saturday citing former American officials from the final weeks of 2010 till the end of 2012, the Chinese killed up to 20 CIA sources.

Read: Department Of Justice Indicted Russian Spies, Hackers For Massive Breach In 2014

Officials also said the number of U.S. assets lost to China were comparable to the loss of assets in the Soviet Union and Russia because of the two infamous spies Aldrich Ames and Robert Hanssen.

Ames had joined CIA in 1962 in a low-level position. Over the years, he worked on several interesting cases but financial constraints started building up due to troubles in his personal life. In 1985, Ames went to the Soviet Embassy in Washington D.C. and offered secrets to the KGB — Komitet gosudarstvennoy bezopasnosti, the committee for the security of the Soviet Union — and started receiving money from them. In 1994, Aimes was arrested for spying for Russia and sentenced to life imprisonment without parole, according to the website of CIA.

Hanssen is considered the "most damaging spy" in FBI history for disclosing confidential information about the U.S. to the Soviet Union and Russia. On Feb. 18, 2001, he was arrested and charged with committing espionage on behalf of the intelligence services of the former Soviet Union and its successors, and was also sentenced to prison without the possibility of parole, according to the website of FBI.

China has reportedly attempted to disrupt American spying efforts several times in the past. In 2015, the U.S. pulled out spies from China as a result of a cyber attack that compromised the personal data of 21.5 million government workers, CNN had reported citing a U.S. official.

On that occasion, the U.S. suspected Chinese hackers were responsible for the breach at the U.S. Office of Personnel Management, which exposed the fingerprints of 5.6 million government employees. The hack reportedly had a huge impact on U.S. national security, in part because the hacked data included information from U.S. government forms used for security clearances.

In April, ahead of President Donald Trump and Chinese President Xi Jinping's summit, a hacking group that pursues Chinese government interests broke into the website of the National Foreign Trade Council (NFTC). They left a malicious link on web pages where members of NFTC register for upsoming meetings, Reuters reported citing researchers at Fidelis Cybersecurity and a person familiar with the trade group.

The FBI and the NFTC had declined to comment. A spokesperson for the Chinese foreign ministry did not respond to a request for comment.