The prisoners, suspected of involvement in such acts as the September 11, 2001, attacks, the 1998 US embassy bombings in Kenya and Tanzania, and the 2002 nightclub bombings in Bali, Indonesia, are being held indefinitely and incommunicado, with no access to counsel, Human Rights Watch alleged.

According to the New York-based monitor, US government officials, speaking anonymously to journalists, have suggested that some of the detainees have been tortured or otherwise seriously mistreated in CIA custody.

"President (George W.) Bush speaks about bringing terrorists to justice, yet not one of these suspects has actually been brought to justice," said John Sifton, the watchdog's terrorism and counter-terrorism researcher.

"The Bush administration has severely compromised the chances of prosecuting terrorist suspects by holding them illegally, and reportedly subjecting some of them to torture and other mistreatment," Sifton said.

Human Rights Watch said the list of 26 names was incomplete and that there were likely many other detainees being held without legal rights, and without being reported to or seen by the International Committee of the Red Cross.

Among those on the list were Khalid Sheikh Mohammed, an alleged mastermind of the September 11 attacks and the attack on the USS Cole in Yemen in 2000, and Ramzi bin al-Shibh, a suspected al-Qaeda conspirator and former roommate of one of the September 11 hijackers.

None of the 26 people on the list have been arraigned or criminally charged, Human Rights watch said.

Reports of clandestine CIA interrogation centers in Europe and transport flights for terrorist suspects emerged last month, along with suggestions of on-board torture sessions.

The US State Department said Wednesday it would provide a timely and forthright answer to a European Union letter expressing concern over the reports.

US officials have refused to confirm or deny the existence of such secret facilities but defended in general terms the use of tough tactics in the war on terror.