The European Union has formally requested answers from the Bush administration about reports of secret U.S.-run prisons for terrorism suspects in Europe, and the United States will reply "to the best of our ability," the State Department said Wednesday.

Britain, which holds the revolving presidency of the EU, sent a two-paragraph letter to Washington on Tuesday, after weeks of mounting outcry in Europe over reports that the CIA has detained and interrogated terrorism prisoners in Soviet-era compounds in Eastern Europe.

State Department spokesman Sean McCormack would not answer questions about whether the sites exist or whether the CIA used European airports and airspace to transport suspects. He also would not say whether the U.S. response to the Europeans will definitively answer those questions, nor whether the U.S. response will be made public.

"We will ... endeavor to respond to this letter to the best of our ability, in a timely and forthright manner," he said.

McCormack set no deadline for a reply. Secretary of State Condoleezza Rice will be in Europe next week, including a stop in Romania, which is one of the nations identified by Human Rights Watch as a likely site of a secret detention camp.

The Washington Post first reported on the alleged prison network Nov. 2.

It would be illegal for the U.S. government to hold prisoners in isolation and difficult conditions in secret prisons in the United States. It has long been assumed that the CIA operates overseas sites to get around U.S. law and to keep terrorism suspects out of the jurisdiction of U.S. courts.

Concerns about alleged CIA activities in Europe have led to investigations in a half-dozen countries. The CIA has declined to comment on the investigations, and the White House and State Department have not confirmed any of the allegations.

"I think as Dr. Rice goes round European capitals, she may find a number of embarrassing questions," said Sir Menzies Campbell, foreign affairs spokesman for Britain's rival Liberal Democrat party.

He accused the U.S.-allied government of Prime Minister Tony Blair of adopting a "hear no evil, see no evil" attitude toward the issue of detainee treatment and transport.

"I don't believe that that is adequate in the light of what we know," Campbell told BBC radio.

"If, in fact, people are being moved from a jurisdiction where torture is illegal to a jurisdiction where torture is permissible, that seems to me to be wholly contrary to international law," Campbell added. "If we are allowing facilities for aircraft carrying out these actions, we are at the very least facilitating and we may even be complicit in it."

McCormack gave no indication that the United States will try to put the allegations to rest before Rice leaves Washington.

"The secretary will look forward to having whatever discussions concerning this matter do arise in her meetings in Europe," McCormack said.

Neither the United States nor Britain has released the text of the letter. Other nations have also made independent attempts to ask the United States if the allegations are true, and McCormack said there have been no replies yet.

A British civil liberties group on Wednesday asked the chiefs of 11 police forces to investigate claims that secret U.S. prisoner flights have landed in Britain.

On Monday, the European Union's top justice official warned that EU nations could lose voting rights in the 25-nation bloc if they hosted a clandestine detention center.

A secret jail would violate the European Convention on Human Rights, EU Justice and Home Affairs Commissioner Franco Frattini said.

Prisoner transport flights without the knowledge of local authorities would violate international aviation agreements, he said.