The White House and the leaders of the intelligence committees in Congress are rejecting National Security Agency-contractor Edward Snowden's plea for clemency.

"Mr Snowden violated US law," White House adviser Dan Pfeiffer said on Sunday about the former systems-analyst-turned-fugitive who has temporary asylum in Russia.

"He should return to the US and face justice," Pfeiffer said, adding when pressed that no offers for clemency were being discussed.

Snowden made the plea in a letter given to a German politician and released on Friday. In his one-page typed letter, he asks for clemency for charges over allegedly leaking classified information about the NSA to the news media. "Speaking the truth is not a crime," Snowden wrote.

Snowden's revelations, including allegations that the US has eavesdropped on allies such as German Chancellor Angela Merkel, have led to calls by allies to cease such spying, and moves by Congress to overhaul US surveillance laws and curb the agency's powers.

But the head of the Senate Intelligence Committee said if Snowden had been a true whistle-blower, he could have reported it to her committee privately.

"That didn't happen, and now he's done this enormous disservice to our country," said Democratic Sen. Dianne Feinstein. "I think the answer is no clemency."

The chairman of the House Intelligence Committee, Rep. Mike Rogers, called clemency for Snowden a "terrible idea".

"He needs to come back and own up," said Rogers, a Republican. "If he believes there's vulnerabilities in the systems he'd like to disclose, you don't do it by committing a crime that actually puts soldiers' lives at risk in places like Afghanistan."

Rogers contended that Snowden's revelations had caused three terrorist organizations to change how they communicate.

Both lawmakers addressed claims that US President Barack Obama did not realize Merkel's personal phone was being tapped.

Rogers implied that he didn't believe the president, or European leaders, who claimed they were shocked by Snowden's allegations.

"I think there's going to be some best actor awards coming out of the White House this year and best supporting actor awards coming out of the European Union," he said.

"Some notion that ... some people just didn't have an understanding about how we collect information to protect the United States to me is wrong."

Feinstein said she didn't know what the president knew, but said she intended to conduct a review of all intelligence programs to see if they were going too far.

"Where allies are close, tapping private phones of theirs ... has much more political liability than probably intelligence viability," she said.

Feinstein and Rogers have taken grief for defending the NSA.

Feinstein's committee produced a bill last week that she says increases congressional oversight and limits some NSA powers under the Foreign Intelligence Surveillance Act. Privacy advocates say the measure codifies the agency's rights to scoop up millions of Americans' telephone records.

Former NSA and CIA director Mike Hayden said it was possible Obama did not know about the alleged Merkel phone tapping.

But he said it was "impossible" that Obama's top staffers were unaware. "The fact that they didn't rush in to tell the president this was going on points out what I think is a fundamental fact: This wasn't exceptional. This is what we were expected to do."