US whistleblower Edward Snowden said the US government had been hacking into computers in Hong Kong and on the Chinese mainland for years.

In an hour-long interview with the South China Morning Post, the former Central Intelligence Agency analyst revealed that the US National Security Agency (NSA) had been hacking computers in Hong Kong and on the mainland since 2009.

Among the targets were the Chinese University of Hong Kong and public officials, businesses and students in the SAR, and the NSA also hacked mainland targets, the newspaper reported.

Snowden believed there had been more than 61,000 NSA hacking operations globally, with hundreds of targets in Hong Kong and on the mainland.

Snowden has been in Hong Kong since May 20 when he fled his home in Hawaii to take refuge. He said he will stay in the city "until I am asked to leave".

"I have had many opportunities to flee HK, but I would rather stay and fight the US government in the courts, because I have faith in HK’s rule of law."

The 29-year-old also said he is in constant fear for his own safety and that of his family.

Snowden said he was releasing the information to demonstrate "the hypocrisy of the US government when it claims that it does not target civilian infrastructure."