The Associated Press said Monday the US Justice Department had secretly obtained two months of phone records from its news operations, calling it a "massive and unprecedented intrusion."

The US news agency protested the seizure in a letter to Attorney General Eric Holder saying "there can be no possible justification for such an overbroad collection of the telephone communications of The Associated Press and its reporters."

The AP said in a news dispatch that the records may be sought in a criminal investigation into leaked information contained in a May 2012 AP story about a foiled terror plot.

"The story disclosed details of a CIA operation in Yemen that stopped an Al-Qaeda plot in the spring of 2012 to detonate a bomb on an airplane bound for the United States," the AP article said.

The AP's Erin Madigan White said in a blog posting that the AP learned Friday that US authorities had "secretly obtained telephone records for more than 20 separate telephone lines assigned to AP journalists and offices, including cell and home phone lines."

AP chief executive Gary Pruitt said in the letter to Holder that the news agency objected "in the strongest possible terms to a massive and unprecedented intrusion... into the newsgathering activities of The Associated Press."

Pruitt said the records "cover a full two-month period in early 2012" and included the AP general phone number in New York, AP bureaus in New York; Washington; Hartford, Connecticut; and at the House of Representatives.

"This action was taken without advance notice to AP or to any of the affected journalists, and even after the fact no notice has been sent to individual journalists whose home phones and cell phone records were seized by the Department," he said in the letter.

"We regard this action by the Department of Justice as a serious interference with AP's constitutional rights to gather and report the news."

Pruitt said the agency was studying its legal options, and called on the Justice Department to "immediately return to the AP the telephone toll records that the Department subpoenaed and destroy all copies."

The American Civil Liberties Union in a statement blasted the government for accessing the news service's records.

"The media's purpose is to keep the public informed and it should be free to do so without the threat of unwarranted surveillance," said Laura Murphy, director of the US civil right group's Washington legislative office.

White House spokesman Jay Carney referred reporters' questions on the matter to the US Justice Department.

The US attorney's office in Washington, which is part of the Justice Department, in a statement sent to AFP, did not specifically mention the AP but stated that it follows specific laws and regulations in seeking records of media organizations.

"Those regulations require us to make every reasonable effort to obtain information through alternative means before even considering a subpoena for the phone records of a member of the media," the statement said.

"We must notify the media organization in advance unless doing so would pose a substantial threat to the integrity of the investigation. 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."

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