Washington  - Sixty-one detainees once held at the Guantanamo Bay, Cuba, prison are believed to have returned to terrorism after being released, the US Defence Department said Tuesday.

Pentagon spokesman Geoff Morrell said intelligence officials have confirmed that 18 have returned to terrorism while 43 are "suspected of returning to the fight."

The numbers represent an increase from 7 per cent to 11 per cent of released detainees suspected or known to have returned to terrroism, Morrell said.

"There clearly are people who are being held at Guantanamo who are still bent on doing harm to America - Americans and our allies," Morrell said. "So there will have to be some solution for the likes of them."

President-elect Barack Obama has pledged to close Guantanamo prison after taking office on Tuesday, and will reportedly issue an executive order to shutter the facility on his first full day in office on Wednesday.

But Obama acknowledged in an interview with ABC News over the weekend that the complex and legal and security issues mean it will take at least months to close the facility.

"I think it's going to take some time and our legal teams are working in consultation with our national security apparatus as we speak to help design exactly what we need to do," he said.

Defence Secretary Robert Gates has already begun the process of reviewing how to close Guantanamo and deal with the remaining 250 detainees, only a handful of which have been charged with crimes.

The Pentagon has already identified an additional 60 inmates who can be transferred or released to another country, but no government has stepped forward to take them, indicating the difficulty of shutting down the prison camp.

It was unclear whether Obama would order a suspension of the trials under President George W Bush's military commissions. (dpa)