The White House is in conversation with Hong Kong over the extradition of whistleblower Edward Snowden, according to reports.
The Reuters news agency reported that a senior US official said relations between the US and Hong Kong will be "complicated" if its extradition request is resisted.
Snowden revealed secret government spying programmes, and has been charged with espionage by US authorities.
As a provisional arrest warrant was issued and Hong Kong authorities asked to detain him, Wikileaks founder Julian Assange jumped to his defence in a statement circulated online.
US prosecutors have filed a criminal complaint, charging Mr Snowden with three offences including unauthorised communication of national defence information, which comes under the Espionage Act, and theft of government property.
He is also charged with willful communication of classified communications intelligence information to an unauthorised person.
All three crimes listed carry a maximum 10-year prison penalty.
Assange, who is holed up in the Ecuadorian embassy in London, has urged the world to "stand with" Mr Snowden.
Wednesday marked a year since Assange sought refuge in the embassy to avoid extradition to Sweden, where he is wanted for questioning over allegations that he raped one woman and sexually molested another.
In a statement he said: "The US government is spying on each and every one of us, but it is Edward Snowden who is charged with espionage for tipping us off," he said.
"The word 'traitor' has been thrown around a lot in recent days. But who is really the traitor here?"
The former CIA technician, who has worked for America's National Security Agency (NSA), leaked details of American telephone and internet surveillance programmes.
He revealed the existence of a surveillance system called Prism that was set up by the NSA to track the use of the internet directly from ISP servers.
The NSA and FBI have said that the secret programme provided "critical leads" in preventing "dozens of terrorist events" - although some terror experts dispute the claims.
President Obama has also said the programmes were carried out with "systems of checks and balances" and overseen by the courts and the US Congress.
The Prism revelations sparked outcry in the UK when The Guardian reported that the GCHQ eavesdropping agency had been accessing information about British citizens through Prism.
Mr Snowden fled to Hong Kong on May 20 after copying the last set of documents he intended to disclose at the NSA's office in Hawaii.
Sky News Asia correspondent Mark Stone said the move marks the official start of government attempts to bring him back to the US.
"We are yet to hear from the Hong Kong police and authorities on whether or not they will act on the request by the Americans to arrest Edward Snowden.
"It’s my understanding that they know exactly where he is. The Americans haven’t yet asked for his extradition, they have simply asked the authorities to arrest him."
There are reports a private plane is on standby to take Mr Snowden from Hong Kong to Iceland, where he hopes to get asylum.
The latest documents from Mr Snowden claim to show that British spies have secretly accessed fibre-optic cables carrying emails, Facebook messages and other communications.
The Guardian reports that GCHQ can analyse data from the network of cables that carry global phone calls and internet traffic under an operation codenamed Tempora.
It claims that communications between innocent people are being processed, as well as those from people marked out as security threats.
"It's not just a US problem," Mr Snowden told The Guardian.
"The UK has a huge dog in this fight. They (GCHQ) are worse than the US."
Mr Snowden worked for the NSA as an employee of various outside contractors, including Dell and Booz Allen Hamilton.
"I can't in good conscience allow the US government to destroy privacy, internet freedom and basic liberties for people around the world with this massive surveillance machine they're secretly building," Mr Snowden previously told The Guardian.