"The location of these prison camps must be made public," said German Socialist Wolfgang Kreissl-Doerfler, who sits on a special EU assembly committee investigating the CIA's alleged operation of secret prisons in Europe, AP reports.
"We need to know if there has been any complicity in illegal acts by governments of EU countries or states seeking EU membership."
Bush did not say where the facilities were located, but the European Parliament's investigation has focused on allegations that several secret prisons were run in Eastern Europe.
The EU parliament's investigation's deputy leader, British Liberal Democrat Sarah Ludford, said Bush's admission "exposes not only his own previous lies. He also exposes to ridicule those arrogant government leaders in Europe who dismissed as unfounded our fears about extraordinary rendition."
The investigation has already yielded an interim report that says the CIA or other U.S. services have been directly responsible for the abduction and detention of terror suspects in Europe and their transport to American custody following the Sept. 11, 2001, terror attacks on the United States.
However, the committee has so far failed to find hard evidence of secret prisons in Europe. Graham Watson, leader of the Liberal Democrats in the EU assembly, said Bush's acknowledgment will "bring a new interest and momentum to the work of the European Parliament's committee."
A parallel investigation by the Council of Europe, the continent's top human rights organization has also not come up with hard evidence, but has accused 14 European nations of colluding with U.S. intelligence in human rights abuses to help the CIA spirit terror suspects to illegal detention facilities.
The allegations that the CIA held and interrogated terror suspects at Soviet-era compounds in Eastern Europe and transported them through European territory in breach of human rights treaties were first reported last year.
"Kidnapping people and torturing them in secret, however tempting the short-term gain may appear to be, is what criminals do, not democratic governments," said Rene van der Linden, president of the Council of Europe's Parliamentary Assembly, which conducted the investigation.
"In the long term, such practices create more terrorists and undermine the values we are fighting for," he said.
European Union lawmakers and civil rights campaigners have long called on U.S. officials to admit the United States used a network of secret prisons and has transferred prisoners between them on covert flights as part of the U.S.-led war on terror.
In July, the EU Parliament voted to continue its investigation for another six months. Its aim is to determine whether European countries have colluded in human rights violations and breached the continent's civil liberties treaties.
Bush said Wednesday that a small number of high-value detainees - including the mastermind of the Sept. 11 attacks, Khalid Sheik Mohammed - had been kept in CIA custody. In total 14 men were transferred from the secret prisons to the U.S. military base in Guantanamo Bay, Bush said, just hours after the information was declassified.
Bush said that interrogation techniques used at the prisons were tough, but did not constitute torture.
Human Rights Watch has previously identified EU member Poland and Romania, which is set to join the EU next year, as possible sites of secret prisons, a charge both countries have repeatedly denied.
"There should be no Russian who goes to sleep without wondering if they're going to get their throat slit in the middle of the night,” Milley said