WASHINGTON (Reuters) - A U.S. senator has asked the White House to explain how and why the name of an al Qaeda informant was leaked to the press, amid concerns it had hurt the war on terror, a letter from the lawmaker shows.

A Pakistani intelligence source said on Friday that U.S. officials confirmed the name of captured al Qaeda suspect Mohammad Naeem Noor Khan while he was still cooperating with Pakistani authorities as part of a sting operation against Osama bin Laden's al Qaeda network.

It is not clear who originally disclosed Khan's name, which first appeared in The New York Times last Monday and was then confirmed by U.S. officials.

His unmasking triggered criticism across the political spectrum, as well as speculation about the motives behind the leak. Security and terrorism are top issues for both parties in this year's U.S. presidential elections.

"I respectfully request an explanation ... of who leaked this Mr. Khan's name, for what reason it was leaked, and whether ... reports that this leak compromised future intelligence activity are accurate," Senator Charles Schumer, a Democrat from New York, wrote in a letter to White House domestic security adviser Frances Townsend on August 8.

A copy of the letter was obtained by Reuters on Monday.

White House spokesman Scott McClellan suggested at a news briefing on Monday that making the name public was inappropriate, but gave no details on how it happened or the repercussions it might have.

"It is important that we recognise that sometimes there are ongoing operations under way. And as we move forward on capturing or bringing to justice al Qaeda members, we need to keep that in mind," he said.

PONDERING MOTIVES

Information from computer expert Khan led the United States to issue a high alert at financial institutions against a possible al Qaeda attack earlier this month, and led Britain to arrest 12 al Qaeda suspects.

Terrorism experts said the reasons for the release of Khan's name could range from a judgment error to a sophisticated ploy designed to put al Qaeda on edge about the extent to which the network has been infiltrated by moles.

Leon Fuerth, Vice President Al Gore's former national security adviser, said: "I can't imagine that this produces any other consequence than to shoot us in the foot" in terms of undermining a sting operation, scaring off future informants and hurting future intelligence cooperation with allies.

One former senior U.S. intelligence official said he suspected a political motive.

"I don't think that the U.S intelligence community has shown enough creativity over the last few years for anyone to think of anything as smart as misdirection, or trying to send signals to al Qaeda," he said.

Republican Senator George Allen of Virginia also questioned the release of Khan's name on television on Sunday, saying: "In this situation, in my view, they should have kept their mouth shut and just said, 'We have information, trust us.'"

National security adviser Condoleezza Rice said it was a hard line to draw between giving the public too much or too little information about terrorist threats.

"We did not, of course, publicly disclose his name," Rice said on Sunday, adding that it had been given "on background."

Khan's capture was part of a Pakistani crackdown, which began a month ago and has dealt al Qaeda a major blow.

Commentary:
Rice is an idiot. Why was it necessary to tell the press anything about this guy? The only possible reason for leaking this story to the press is to take luster off of the Kerry campaign after the Democrat Convention and put the spot light back on terrorism. Surely harming national security isn't a prerequisite of being a national security advisor, but keeping her boss in his job is.

The polls show voters who are undecided move to Bush's side whenever terrorism becomes a headline story (even when every reported is bogus) and Bush knows that's his only ace card so he plays it every chance he gets. Shame on him.

Leaking classified information is a crime so this is another impeachable offense. Also, since this leak helps so©called terrorists it also aided the enemy during a time of war which is treason.