WASHINGTON (Reuters) - The number of former Guantanamo Bay prison inmates who are suspected of having returned to fighting for militants doubled to 12 in the six months through January, the Obama administration said on Monday.

A soldier stands guard in a tower overlooking Camp Delta at Guantanamo Bay naval base in a December 31, 2009 file photo provided by the US Navy. REUTERS/US Navy/Spc. Cody Black/Handout via Reuters

The increase could fuel Republican attacks on Democratic President Barack Obama’s plan to close the U.S. military prison in Cuba, which has come to symbolize aggressive detention practices following the Sept. 11, 2001 attacks and opened the United States to accusations of torture. Most detainees have been held without trial for more than a decade.

The closure plan, drawn up by the Pentagon and which requires approval by Congress, proposes 13 potential sites on U.S. soil to hold 30-60 detainees in maximum-security prisons.

According to figures released by the Office of the Director of National Intelligence (ODNI), as of Jan. 15 the United States also had confirmed that seven out of 144 Guantanamo prisoners who were freed since Obama took office in January 2009 have returned to fighting.

That was up from six since the ODNI’s previous release last July. The ODNI report is released every six months and does not give details on where or for which groups the former detainees are confirmed or suspected to be fighting.

The ODNI figures showed that 111 of 532 prisoners released by the Republican administration of President George W. Bush are confirmed to have returned to the battlefield, with 74 others suspected of doing so. Under Bush, suspected militants were rounded up overseas as the United States became embroiled in wars in Afghanistan and Iraq and imprisoned at Guantanamo.

The closure plan faces strong opposition from lawmakers who do not want detainees transferred to the United States. The United States took control of part of Guantanamo Bay in Cuba in 1903 under a treaty with the Havana government.

Obama has pressed the Republican-led legislature to give his proposal a “fair hearing” and said he did not want to pass the issue to his successor in January. He is also considering executive action to close the facility.

Republican Representative Ed Royce, chairman of the House of Representatives Foreign Affairs Committee, introduced a measure that would force the administration to publicize plans for transfers from Guantanamo.

Four other Republicans, Senators Richard Burr, Kelly Ayotte, Tom Cotton and presidential hopeful Marco Rubio, also introduced a bill that would bar Obama from returning the naval base at Guantanamo to Cuba without authorization from Congress.