WASHINGTON, April 30, 2009 (AFP) - US officials planning the closure of Guantanamo prison are weighing the cases of about 50 to 100 detainees who cannot be tried or released, Defense Secretary Robert Gates said Thursday.

"The question is what do we do with the 50 to 100 -- probably in that ballpark -- who we cannot release and cannot try," Gates told a Senate hearing.

"I think that question is still open," Gates said when asked about President Barack Obama's plans to shut down the controverial "war on terror" prison.

His comments made clear that some inmates might have to be detained further even after the controversial prison at Guantanamo Bay is closed as ordered by Obama.

The US administration is closely reviewing the files of about 240 detainees held at the center to determine who could be transferred to other countries or tried in US civilian courts or special military tribunals set up under former president George W. Bush, Gates said.

About 60 detainees have been cleared of wrongdoing and the previous administration had planned to charge about 80 of the detainees.

The administration was asking Congress for about 50 million dollars to help cover the costs of possible further detention for some of the inmates, who officials say might be held at military prisons on US soil.

Separately, the Justice Department was requesting 30 million dollars to help pay for the effort to review the detainees' cases, Gates told the Senate Appropriations Committee.

Lawmakers warned Gates at the hearing that local governments across the country had already expressed opposition to having any detainees transferred to prisons in their communities.

"I fully expect to have 535 pieces of legislation before this is over saying 'not in my district, not in my state,'" Gates said, referring to the total number of representatives and senators in Congress.

"And we'll just have to deal with that when the time comes."