Two Yemeni men say they were held in solitary confinement in secret, underground U.S. detention facilities in an unknown country and interrogated by masked men for more than 18 months without being charged or allowed any contact with the outside world, Amnesty International (search) charged Wednesday.

Amnesty and human rights lawyers argued that the report added to long-standing claims that the United States has held "secret detainees" in its war on terror.

"We fear that what we have heard from these two men is just one small part of the much broader picture of U.S. secret detentions around the world," Sharon Critoph, a researcher at Amnesty International who interviewed the men in Yemen, told The Associated Press by telephone.

Navy Lt. Commander Flex Plexico (search), noting that it was difficult to respond to a report he hasn't seen said, "We have said many times that the Department of Defense does not engage in the practice of renditions" — the transfer of terror suspects to third countries without court approval.

Plexico, a spokesman for the department, said it was important to note that training manuals of al Qaeda terrorist network "emphasize the tactic of making false abuse allegations."

U.S. officials have denied allegations of secret detention facilities, saying they hold terror suspects only at the U.S. Naval base in Guantanamo Bay, Cuba, and in Iraq and Afghanistan.

In June, U.S. officials denied a suggestion from the U.N.'s special expert on torture, Manfred Nowak, that some undeclared holding areas could include American ships cruising international waters. Others have suggested "high-value" detainees could be held secretly in Diego Garcia (search), a British-held island in the Indian Ocean that the United States rents as a strategic military base.

Lawyers who represent detainees at Guantanamo have long believed that the CIA or other U.S. government agencies have used clandestine jails for terror suspects.

"The fact that there are underground CIA facilities somewhere where people are being tortured has been known for a while," Michael Ratner of the U.S. Center for Constitutional Rights in New York City told the AP in a telephone interview.

In the report released in Washington, Amnesty said it interviewed Salah Nasser Salim Ali and Muhammad Faraj Ahmed Bashmilah in a jail in Yemen in late June. The group also spoke to a Yemeni government official who said the men were being held in that country only because it was a condition of their release from U.S. custody.

Ali told the rights group that he was originally detained in Indonesia in August 2003 and then flown several days later to Jordan; Bashmilah said he was detained in Jordan in October 2003 while on a trip to visit his mother.

Both men claimed they were tortured by Jordanian intelligence agents for four days and then flown to what they believe were underground jails in an unknown location.

Once there, they were held in solitary confinement for more than 18 months, interrogated daily by U.S. guards and blared Western music all day and night. No charges were ever filed against them, they said.

The men said their first jail was underground, surrounded by high walls and that it took more than 4 hours to fly there from Jordan. After six to eight months they were transferred to a modern prison run by U.S. officials a three-hour plane journey away that also appeared to be underground.