Cyberattacks on China that originate in the United States may be partly backed by the US government, Chinese experts have said, a situation they would like Washington to explain.

Their call came after US National Security Agency whistle-blower Edward Snowden told a Hong Kong newspaper that US spies have hacked into Chinese mobile phone companies and a backbone Chinese network to steal text messages and data of millions of Chinese citizens.

The South China Morning Post revealed Snowden's claims on Sunday, a few hours before the former contractor for the US National Security Agency left Hong Kong.

"The scale and scope of US monitoring of China's Internet information is shocking, it is beyond expectation," said Jia Xiudong, a senior researcher of US studies at the China Institute of International Studies.

"The US is the largest source of foreign cyberattacks on China. Now Snowden's claims show that many such attacks may be backed by the US government."

Foreign Ministry spokeswoman Hua Chunying said China is concerned with reports of US government hacking on China and has made representations to the US.

The National Computer Network Emergency Response Technical Team/Coordination Center of China has told China Daily that in the first five months of this year, 13,408 overseas trojan horses or bot control servers — two popular hacking tools — hijacked around 5.63 million mainframes in China. Of those, 4,062 US-based control servers hijacked 2.91 million mainframes in China.

Snowden said that "the NSA does all kinds of things like hacking Chinese cellphone companies to steal all of your SMS data". The NSA also made sustained attacks on network backbones at Tsinghua University and computers at the Hong Kong headquarters of telecom service provider Pacnet, he said.

In one single day in January, at least 63 computers and servers at Tsinghua University were hacked by the NSA, he said.

Tsinghua is home to one of the Chinese mainland's six major backbone networks, the China Education and Research Network, known as CERNET, from where Internet data from millions of Chinese citizens could be mined.

Pacnet owns one of the most extensive fiber-optic submarine cable networks. It recently signed major deals with China's top mobile phone companies, and owns more than 46,000 kilometers of fiber-optic cables. The cables connect its regional data centers across the Asia-Pacific region, including the Chinese mainland, Hong Kong, Taiwan, Japan, South Korea and Singapore.

The Post said Snowden provided the information on June 12. The documents listed operational details of specific attacks on computers, including Internet protocol addresses, dates of attacks and whether a computer was still being monitored remotely.

"What is the connection between Tsinghua's CERNET and terrorism?" Jia asked.

"Obviously US Internet attacks against China are not for anti-terrorism efforts, as Washington has argued, but for other reasons that might include commercial and strategic motives," Jia said. "The US owes an explanation to China, and the world."

However, Jia said Washington might, as always, avoid facing the problem and he did not expect it to stop hacking China.

"(Snowden's) claims sounded the alarm for Beijing to beef up Internet security efforts," he said.

A commentary published on Sunday by Xinhua News Agency said Snowden's disclosures of US spying activities in China have "put Washington in a really awkward situation''.

"Washington should come clean about its record first," it said. "It has to share with the world the range, extent and intent of its clandestine hacking programs."

The Hong Kong government said on Sunday that Snowden has left Hong Kong voluntarily and that Hong Kong has updated Washington with the situation.

The post said Snowden boarded a flight for Moscow on Sunday and his final destination may be Ecuador or Iceland. It did not give any source of the information. Other reports said Cuba, Ecuador or Venezuela may be his choice of destination.

Shi Yinhong, a senior expert on US studies with Renmin University of China in Beijing, said the Snowden case must be handled properly to avoid hurting ties between China and the US.