One of America's leading journalists believes that he is being monitored by the US National Security Agency (NSA).

Brian Ross, chief investigative correspondent for ABC News, wrote in his blog that a senior source warned him that his phone calls were being monitored in an attempt to identify his sources. 

Ross was one of a number of journalists who broke the story of CIA jails being set up in Eastern Europe.

"It's time for you to get some new cell phones, quick," the source warned in an in-person conversation.

"Other sources have told us that phone calls and contacts by reporters for ABC News, along with The New York Times and The Washington Post, are being examined as part of a widespread CIA leak investigation."

The NSA hit the headlines last week when it was revealed that it was compiling the world's largest database of calls made by logging every telephone number dialled in the US since 11 September 2001.

Michael J. Copps, a commissioner at the Federal Communications Commission, is calling for an investigation into the NSA's activities.

"There is no doubt that protecting the security of the American people is our government's number one responsibility," said Copps.

"But in a digital age where collecting, distributing and manipulating personal information is as easy as a click of a button, the privacy of our citizens must still matter."