HONG KONG (AFP) - The United States pressed for Edward Snowden's formal extradition from Hong Kong as the former spy fought back with new allegations aired on Sunday about the far-reaching extent of US cyber-espionage in China.

Documents given by the former intelligence contractor to Hong Kong's Sunday Morning Post could embarrass the United States in Beijing, just as the White House demands the return of a man that many in Washington are calling a "traitor".

The United States slapped an arrest warrant on Snowden on Friday, and White House National Security Advisor Tom Donilon said the charges "present a good case for extradition under the treaty, the extradition treaty between the United States and Hong Kong".

"Hong Kong has been a historically good partner of the United States in law enforcement matters, and we expect them to comply with the treaty in this case," he told CBS Radio News on Saturday.

There was no immediate comment from authorities in Hong Kong, but the government and police have said that the law will take its course. And that, experts say, could take months if not years if Snowden appeals against any extradition ruling. Beijing, meanwhile, has the right ultimately to intervene.

The Sunday Morning Post said the 30-year-old remained "safe" in Hong Kong and had not been detained by police after he was charged in the United States with theft and espionage.

Abandoning his well-paid job as an intelligence technician in Hawaii, Snowden came to Hong Kong on May 20 with a cache of documents detailing the reach of National Security Agency (NSA) operations around the world.

In the latest revelations published by the Sunday Morning Post, the former NSA contractor said the US government agency was hacking Chinese mobile phone companies to gather data from millions of text messages.

Snowden said US spies have also hacked the prestigious Tsinghua University in Beijing - home to one of six "network backbones" that route all of mainland China's Internet traffic - and the Hong Kong headquarters of Pacnet, which operates one of the Asia-Pacific region's largest fibre-optic networks.

The claims followed soon after a report in the Guardian in which he claimed the British government's electronic eavesdropping agency GCHQ had gained secret access to fibre-optic cables carrying global Internet traffic and telephone calls, and was sharing the information with the NSA.

"The NSA does all kinds of things like hack Chinese cell phone companies to steal all of your SMS data," Snowden said in the Post interview, which the newspaper said was conducted on June 12 and released after it had scrutinised and clarified his claims.

The Post said Snowden had provided documents listing operational details of specific attacks on Chinese and Hong Kong computers, including internet protocol (IP) addresses, over a four-year period.

Chinese government data shows almost 900 billion text messages were exchanged in the mainland in 2012. Texting remains the preferred mode of communication for many, rather than voice calls.

The Post has previously quoted Snowden as saying there have been more than 61,000 NSA hacking operations globally, targeting powerful network backbones that can yield access to hundreds of thousands of individual computers.

Snowden said in the latest interview that Tsinghua University, which counts China's President Xi Jinping and his predecessor Hu Jintao among its graduates, was the target of extensive NSA hacking.

The university, whose network backbone handles Internet data from millions of Chinese citizens, was breached as recently as January, he was quoted as saying.

According to the Post report, the NSA in 2009 also attacked Pacnet, whose fibre-optic network stretches across 46,000 kilometres in 13 countries ranging from Singapore to Japan via Hong Kong and China.

In turn, the Pacnet network links up across the Pacific to the United States, and the Post quoted experts as saying it would be straightforward for the NSA to patch into communications from the US side.

The drip-feed of revelations has embarrassed US President Barack Obama's administration, which has been forced to defend the gathering of huge amounts of telephone and Internet data from private users around the world.

Administration officials say the surveillance has helped thwart up to 50 planned extremist attacks, some of them on US soil, by allowing US agents to track calls and messages to enemy operatives.