One in five terror suspects released from the Guantanamo Bay military prison has returned to carrying out militant activities, according to a classified Pentagon report.

The finding, which is expected to stoke an already fierce debate over U.S. President Barack Obama's plan to close the facility, reflects an upward trend in the recidivism rate. However, human rights activists who advocate closing the prison have questioned the validity of the numbers.

Early last year, the Pentagon reported that the rate of released detainees returning to militancy was 11 per cent. In April, it was 14 per cent. The latest figure was 20 per cent, according to a U.S. official, who spoke on condition of anonymity because the report had not been declassified and released.

Critics of the reports say there is so little information in the assessments that they are nearly impossible to verify independently.

Civil rights advocates say the number of fighters suspected of or confirmed as returning to the battlefield is likely to be much smaller.

Meanwhile, Republicans say the rising number suggests the U.S. detention facility at Guantanamo Bay, Cuba, cannot be closed because that would mean either releasing hardened terrorists back into the fight or moving them into U.S. prisons, which many Americans oppose.

Although Obama said Tuesday that he still wants to close the detention facility, Defence Department press secretary Geoff Morrell suggested Wednesday that such plans were on hold. Congress has put severe limits on the endeavour.

Under significant political pressure, Obama has said he won't release any more detainees to Yemen because of al-Qaeda's grip on that nation. Nearly half of the remaining 198 detainees at Guantanamo Bay are from Yemen.

The attempted Christmas Day attack on a flight from Amsterdam to Detroit has heightened concerns about Yemen because the 23-year-old Nigerian passenger accused of attempting to detonate explosives in the plane claimed to be acting on instructions from al-Qaeda operatives in Yemen.