The Obama administration's decision to seize phone records from the Associated Press was "unconstitutional" and sends a message that "if you talk to the press, we are going to go after you", the news agency's boss Gary Pruitt said Sunday.

AP revealed last week that the Justice Department had obtained two months' worth of phone records of calls made by reporters and editors without informing the organisation in advance. The move was an apparent effort by US officials to identify the source of a story about the CIA foiling an alleged terrorist plot by an al Qaida terrorist affiliate in Yemen.

News of the seizure has caused a political firestorm and comes amid a widening scandal into the Internal Revenue Service's targeting of Tea Party groups over their tax exemptions and the White House's handling of the Benghazi terrorist attack last year.

Speaking on CBS's Face the Nation, Pruitt, AP's president and chief executive officer, said the government's seizure of the phone records was "unconstitutional" and was already clearly harming the press's ability to do its job.

"We don't question their right to conduct these sort of investigations. We just think they went about it the wrong way. So sweeping, so secretly, so abusively and harassingly and over-broad that it constitutes, that it is, an unconstitutional act," he said.

"We are already seeing some impact. Already officials that would normally talk to us and people we talk to in the normal course of newsgathering are already saying to us that they are a little reluctant to talk to us. They fear that they will be monitored by the government. We are already seeing that. It's not hypothetical," said Pruitt.

The government investigation was seemingly triggered by an AP exclusive about a joint US-Saudi spy operation that had foiled a plot involving an improved version of the "underwear" bomb that failed to detonate properly on a Detroit-bound flight on Christmas Day 2009. AP agreed to delay publication after officials cited national security concerns.

Pruitt said he would normally expect dialogue with government officials ahead of any decision to ask for or demand records relating to the news organisation's activities. Those requests would usually be subject to negotiation and if an agreement could not be reached, they would be put before a judge, he said.

In this case, the Justice Department has claimed it made every reasonable effort to obtain the information through alternative means, as is required by law. "Because we value the freedom of the press, we are always careful and deliberative in seeking to strike the right balance between the public interest in the free flow of information and the public interest in the fair and effective administration of our criminal laws," it said in a statement.

Pruitt said he had not received any explanation as to why AP had not been consulted ahead of the seizure. "I really do not know what their motive is. I know what the message being sent is, it's that if you talk to the press, we are going to go after you," he said.

Pruitt said the Justice Department had acted "as judge jury and executioner, in secret".

If the government restricts the "news gathering apparatus" then "the people of the United States will only know what the government wants them to know. And that's not what the framers of constitution had in mind when they wrote the first amendment," Pruitt said.

The White House has denied knowledge of the Justice Department's move. It comes as officials face mounting criticism over an IRS investigation into Tea Party groups. Senate Republican leader Mitch McConnell told NBC's Meet the Press Sunday that the IRS controversy demonstrated a "culture of intimidation" by the administration.