US President George W. Bush blocked a Justice Department probe into a secret program to tap international phone calls and electronic communications of US citizens, Attorney General Alberto Gonzales said.

Testifying before the Senate Judiciary Committee, Gonzales confirmed that Bush put a halt to an enquiry by department lawyers into the National Security Agency (NSA) program, which involved wiretaps without court warrants.

Meanwhile, two US lawmakers challenged the Justice Department on Tuesday to explain what rights the administration had in tapping phones and allegedly leaking information about sensitive investigations to the media.

The department's Office of Professional Responsibility (OPR) was to look into the department's role in the NSA program and the legality of its warrantless eavesdropping.

The electronic surveillance program sparked a firestorm after its existence was first disclosed in December by the New York Times.

In a letter to Gonzales made public Tuesday, Democratic Senator Charles Schumer and Representative William Delahunt pointed to alleged security leaks in the Bush administration.

The letter focused attention on US intelligence efforts and public accountability. Administration officials defended their effort, but the opposition lawmakers questioned one of the most delicate issues now facing the Bush administration.

They particularly pointed to The Washington Times, the conservative US capital daily, as reporting news "that appear to have been classified at the time of disclosure."

The letter asked the Bush administration to explain how such information could be published in newspapers when the country was engaged in a "war on terror."

"We are writing to seek clarification of administration policy regarding classification of national security information, as well as the role of the Department of Justice in investigating possible leaks of such information," the letter stated.

"How many referrals to the Department of Justice of suspended leaks of information have been made in the last five years?" the letter asked.

"What, if any action, has been taken on such referrals."

At the White House, presidential spokesman Tony Snow defended Bush's action as necessary to limit the number of people with access to highly classified information.

"What (Bush) was saying is that in a case of a highly classified program, you need to keep the number of people exposed to it tight, for reasons of national security, and that's what he did," Snow told reporters.

The OPR "was not the appropriate venue" for an enquiry, Snow said. "There was already an appropriate venue for doing this...specified by executive order."

Snow said a legal review of the NSA program is already undertaken every 45 days with the involvement of the attorney general.

Civil rights advocates argue that without court oversight the plan could be abused and trample personal privacy rights.