The US government's secret seizure of Associated Press phone records had a "chilling effect" on newsgathering by the agency and other news organizations, AP's top executive said Wednesday.

"Some longtime trusted sources have become nervous and anxious about talking with us," AP president and chief executive Gary Pruitt said in a speech to the National Press Club.

"In some cases, government employees we once checked in with regularly will no longer speak to us by phone. Others are reluctant to meet in person ... This chilling effect on newsgathering is not just limited to AP.

"Journalists from other news organizations have personally told me that it has intimidated both official and nonofficial sources from speaking to them as well."

Pruitt spoke one month after the US news agency revealed that it had been notified after the fact that the US Justice Department had secret subpoenas of two months of phone records from its news operations.

The AP has said US authorities appeared to have sought out the records as part of a criminal investigation into leaked information contained in a May 2012 AP story about a foiled terror plot.

Pruitt, who previously called the seizure "a massive and unprecedented intrusion" into newsgathering, said the Justice Department "violated its own rules" on how it handles investigations of leaks to news media.

He said the collection of records pertaining to more than 100 journalists was "an overbroad and sloppy fishing expedition" and failed to follow procedures on notification.

Pruitt said that authorities maintained that by notifying the AP ahead of the sweep "it would have tipped off the leaker" but argued "that kind of reasoning would apply in every single case."

This rationale would mean news organizations would never know when its records are being obtained, news sources would become less willing to speak and "the public will only know what the government wants them to know."

The Justice Department has told the AP "that our phone records have been and will continue to be walled off, protected and used for no other purpose other than the leak investigation," Pruitt said.

"We appreciate these assurances. But that does not excuse what they did. We need to make sure it doesn't happen again."

The AP chief said the US administration should reaffirm the right of advance notice to news organizations, and use the courts to adjudicate any disputes on whether certain records are needed.

He also called for a "federal shield law with teeth" to ensure that journalists are not prosecuted for doing their jobs.

"We do not dispute that the government has the right to pursue those who leak classified information," he said.

But he argued that "no one in this country should ever be prosecuted for committing journalism."

Last month, Attorney General Eric Holder said the leak which prompted the seizure of journalist phone records was a "very serious" matter which "puts the American people at risk."

Pruitt said Wednesday however that the AP waited five days before publishing the article, until after it had been assured by US officials that "the national security risk had passed.

The US administration under President Barack Obama has been aggressive in pursuing leaks of secret government information.

Authorities have said they had opened a probe into Edward Snowden, the former government contractor who leaked details about a cast US government electronic surveillance program.

Former CIA officer John Kiriakou was sentenced in January to two and a half years in prison for leaking the name of a secret agent implicated in harsh interrogations of Al-Qaeda suspects.

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