The NSA whistleblower Edward Snowden was condemned by US politicians and threatened with prosecution by the country’s intelligence chief on Sunday after revealing himself as the Guardian’s source for a series of explosive leaks on the NSA and cyber surveillance.

A spokesman for the director of national intelligence, James Clapper, said Snowden’s case had been referred to the justice department and US intelligence was assessing the damage caused by the disclosures.

“Any person who has a security clearance knows that he or she has an obligation to protect classified information and abide by the law,” the spokesman, Shawn Turner, said.

Snowden had top-secret clearance to help run the National Security Agency’s computer systems but he was a contractor, hired by the giant US defence contractor Booz Allen Hamilton. The company issued a statement describing the disclosures as “shocking” and pledging to co-operate with any investigation.

It said: “News reports that this individual has claimed to have leaked classified information are shocking, and if accurate, this action represents a grave violation of the code of conduct and core values of our firm. We will work closely with our clients and authorities in their investigation of this matter.”

There was no immediate reaction from the White House but Peter King, the chairman of the House homeland security subcommittee, called for Snowden’s extradition from Hong Kong. Snowden flew there 10 days ago to disclose top-secret documents and to give interviews to the Guardian.

“If Edward Snowden did in fact leak the NSA data as he claims, the United States government must prosecute him to the fullest extent of the law and begin extradition proceedings at the earliest date,” King, a New York Republican, said in a written statement. “The United States must make it clear that no country should be granting this individual asylum. This is a matter of extraordinary consequence to American intelligence.”

The US has an extradition treaty with Hong Kong, but there are exclusions for political offences.

The Republican head of the House intelligence committee, Mike Rogers, said Snowden had “released just enough information to literally be dangerous”.

But Snowden drew support from civil liberty activists and organisations. Jesselyn Radack, a former justice department attorney who represents whistleblowers, told Reuters: “As a whistleblower myself, this is one of the most significant leakers in my lifetime and in US history.”

Radack said she hoped the case could become “a watershed moment that could change the war on whistleblowers and the broader war on information in our country”.

Thomas Drake, a former NSA executive who famously leaked information about what he considered a wasteful datamining program at the agency, said of Snowden: “He’s extraordinarily brave and courageous.”

Drake was investigated so intensely by the justice department that the longtime analyst was reduced to working at an Apple store until the Obama administration abruptly dropped charges that could have landed him in jail for 35 years.

“It’s an extraordinarily magnanimous act of civil disobedience to disclose the Pandora’s Box of the Leviathan state,” Drake told the Guardian as he returned from a weekend appearance in New York at the Left Forum, where he spoke about whistleblowing and national security.

Drake was returning to Washington by train with Radack. “We’ve had these moments of epiphanies with things like Sandy Hook, where we talk about gun control for a week,” Radack said. “I feel like this is another one of those moments writ large, where the whole country is talking about it and everyone’s pretty much in agreement that the NSA has overstepped.”

Russell Tice, a former NSA analyst who accused the agency in the mid-2000s of overstepping the bounds of its legal surveillance mandate, said: “This guy has more courage than anyone I know.

“The biggest threat to him right now is that the Chinese communists will make a deal with us, a good neighbour deal, to serve him up to you,” Tice said. He encouraged more NSA employees to leak evidence of impropriety in the wake of Snowden’s disclosure.

“I encourage everyone to read the constitution, especially about Probable Cause and  the fourth amendment, and to do the right thing,” Tice said. “I’d say this young man stood up and abided by his oath and the rest are just spinmeisters.”