BAGHDAD - Some prisoners held indefinitely without charge by U.S. forces in Iraq may not be freed or given trials, even though U.S. forces lost the authority to hold them at the beginning of this year, a U.S. military spokesman said.

Iraqi legal experts said the plans -- which would apply to prisoners U.S. forces believe are dangerous or of intelligence value but have not been charged with a crime -- might violate Iraqi law by placing detainees beyond the reach of the courts.

U.S. forces are holding 15,000 prisoners, most of whom have been detained without charge under the authority of a U.N. Security Council resolution which expired on December 31.

Under the terms of a bilateral pact which took effect on January 1, Washington agreed that all its prisoners would either be transferred to Iraqi custody under arrest warrants from Iraqi judges, or freed "in a safe and orderly manner."

The agreement does not mention any mechanism for continuing to hold prisoners without charge, and Iraqi legal experts say there is no such provision under Iraqi law.

But U.S. military spokesman Major Neal Fisher said Washington will ask Iraq not to free prisoners it considers "'radical extremists' or 'enduring threats.'"

"There will be cases in which a detainee falls into the category of high intelligence value or is a high threat but has not violated an Iraqi law, and thus should be released based on the security agreement," he said in an e-mail.

"We seriously desire that they will choose to keep these particular detainees off the streets."

An Iraqi government spokesman was not available to comment on whether Iraq would try to hold former U.S. prisoners without trial. Iraqi legal experts said such measures appear illegal.

ILLEGAL

"Neither the U.S. forces nor the Iraqi government has the right to keep detainees in prison without trial. It is against the Iraqi constitution and Iraqi laws," said Tariq Harib, a leading Iraqi criminal lawyer. "Every detainee should be freed if there is not enough evidence to convict him."

Kamal Abdul-Karim, an investigative judge, said courts are required to order the immediate release of all prisoners in cases where they find insufficient evidence to convict them.

"No one, whether from inside the government or outside it, can exert any pressure on us.... Detainees in prison without enough evidence, we will order their release immediately."

Prime Minister Nuri al-Maliki presented the U.S. pledge to free them or place them under Iraqi law as a milestone.

Over the past year, as violence subsided, the U.S. military has freed prisoners at a faster rate than it detained them, reducing the population from more than 26,000 to about 15,000.

It now says it is going through the remaining cases to determine who can be charged and who must be freed. Prisoners who cannot be charged will be ranked according to how dangerous they are and then released in order of that rank, Fisher said.

He said there was no firm deadline for how long that would take but it could last into 2010. Leaders of Iraq's Sunni Arab minority, which includes most detainees, said that was too slow.

"These statements do not reflect what has been agreed upon," said Abdul-Karim al-Sammaraie, member of parliament from the Sunni Iraqi Islamic Party. "We agreed to steady, gradual release.... We expect the gradual release to continue. By the end of the year prisons should empty of detainees."

(Additional reporting by Khalid al-Ansary; Editing by Jon Boyle)