Around one in five former inmates of the Guantanamo Bay military prison have gone back to extremist activity, a US official said, DPA reported.
The LA Times reported Wednesday that a Pentagon report estimated that 20 per cent of prisoners released from the prison camp on Cuba are suspected of returning to hostile activity against the US.
Officials refused to discuss the classified report, but Pentagon press secretary Geoff Morrell admitted that the recidivism rate among ex-detainees had risen since April when it was put at 14 per cent.
Counting how many ex-prisoners have returned to extremist activity has proved politically controversial. Conservatives say the figures underestimate the real scale of the problem while liberals say previous statistics were inflated.
Then-president George W Bush released around 500 detainees from Guantanamo and President Barack Obama has freed several dozen to date.
Obama has ordered the closure of Guantanamo but this has been delayed. He plans to accommodate the remaining prisons at a facility in Thomson, Illinois.
The Obama administration recently said that transfers of Guantanamo detainees to Yemen would be stopped.
Former Guantanamo inmates are among the leadership of Al Qaeda in the Arabian Peninsula, which claimed responsibility for the attempted bombing of a US airliner on Christmas Day.
White House press secretary Robert Gibbs said he was not sure whether the decision was due to worries ex-prisoners would revert to extremism.
"We never have a plan to transfer anybody either to a home country or to a third country that we have reason to believe will present a security situation for us or for that country," he said.