The Committee to Protect Journalists, in a report based on interviews with dozens of experienced news professionals, said the US president's actions have been in sharp contradiction to his promise of transparency and open government.

"Journalists and transparency advocates say the White House curbs routine disclosure of information and deploys its own media to evade scrutiny by the press," said the report written by former Washington Post executive editor Leonard Downie.

"Aggressive prosecution of leakers of classified information and broad electronic surveillance programs deter government sources from speaking to journalists."

Downie, now a professor of journalism at Arizona State University, said Obama has failed to live up to his pledge to make his administration the most transparent in American history.

Downie added that the Obama administration's "war on leaks and other efforts to control information are the most aggressive I've seen since the Nixon administration, when I was one of the editors involved in The Washington Post's investigation of Watergate."

He said the 30 experienced Washington journalists he interviewed at a variety of news organizations whom I interviewed for this report "could not remember any precedent."

The report on the United States is unusual for the press freedom group, which has completed reports this year on Burma, China, Egypt, Iran, Pakistan, and Tanzania. The only time the United States has been the subject of a CPJ report was 19 years ago, in a study on attacks on immigrant journalists.

The report relates how the US government has conducted more than twice as many criminal prosecutions for alleged leaks of classified information than all the previous administrations combined.

It notes the administration's "Insider Threat Program," which requires federal employees to monitor the behavior of their colleagues, and the use of secret subpoenas to monitor journalists' electronic communications.

Highlighted in the report is the use of the Espionage Act to crack down on news leaks. This was used to prosecute Stephen Jin-Woo Kim, a State Department contract analyst who disclosed information about North Korea's nuclear plans to a Fox News reporter.

The Espionage Act was also used to prosecute US Army private Chelsea Manning for releasing classified information to Wikileaks.

Lucy Dalglish, dean of journalism at the University of Maryland, said the case of Manning, formerly known as Bradley Manning, was a turning point because it is a powerful tool generally reserved for cases of government spying, not news leaks.

"Prosecution under the 1917 Espionage Act is almost their only tool," she said. "They're sending a message. It's a strategy."

New York Times national security reporter Scott Shane, cited in the report, said, "Most people are deterred by those leaks prosecutions. They're scared to death."

White House officials, interviewed for the report, strongly objected being portrayed as against press freedom.

"The idea that people are shutting up and not leaking to reporters is belied by the facts," Obama's press secretary, Jay Carney, told Downie.

But Michael Oreskes, a senior managing editor of The Associated Press, told CPJ that news sources "are looking over their shoulders," due to the stance of the Obama administration.

"Sources are more jittery and more standoffish, not just in national security reporting," Oreskes said. "There's a mind-set and approach that holds journalists at a greater distance."

In a statement accompanying the report, the committee said it was "disturbed by the pattern of actions" which "have chilled the flow of information on issues of great public interest, including matters of national security."

It said these actions "thwart a free and open discussion necessary to a democracy" and called on the Obama administration to "affirm and guarantee that journalists will not be at legal risk or prosecuted for receiving confidential and/or classified information."

The recommendations were sent to Obama in a letter from CPJ chairman Sandra Mims Rowe and executive director Joel Simon.

Christopher Callahan, dean of the Walter Cronkite School of Journalism at ASU, said the mere fact that CPJ chose to investigate the US government's treatment of the press "is a remarkable statement here in the home of the First Amendment."

"US government tactics are increasingly impeding journalists' work and placing a chill on newsgathering that could endanger our democracy," Callahan added.

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