A communal area in an unused portion at Camp VI detention facility for captured al-Qaeda and Taliban militants is shown during a tour at Guantanamo Bay Naval Base, Cuba, Tuesday, Nov. 19, 2013. Charles Dharapak/AP

The United States has sent two detainees held at the Guantanamo Bay detention facility back to their native Sudan, the Defense Department said on Wednesday, the latest transfers in a effort toward eventually closing the controversial prison.

The repatriation of Noor Uthman Muhammed, who is about 46 years old, and Ibrahim Idris, who is roughly 52, to the North African country decreased the prisoner count to 158, Defense Department spokesman Lieutenant Colonel J. Todd Breasseale said in a statement.

"The United States coordinated with the government of Sudan regarding appropriate security measures and to ensure that these transfers are consistent with our humane treatment policy," Breasseale said.

Noor, who asked to be called by his first name, had been held at Guantanamo for nearly nine years when he pleaded guilty in February 2011 to conspiring with Al-Qaeda and providing material support for militants. He admitted he was a weapons trainer at the Khaldan paramilitary camp in Afghanistan from 1996 to 2000. In exchange for a guilty plea and cooperation with prosecutors, he was given a 34-month sentence that ended on Dec. 3.

Idris, who had not been charged with a crime, was ordered freed by a U.S. district judge in early October after his lawyers argued he was too mentally and physically ill to pose a threat. Idris, who is obese and suffers from diabetes and other health problems, was diagnosed with schizophrenia soon after his arrival at Guantanamo in 2002 and has spent a large part of the last decade in a psychiatric ward.

A 2008 U.S. military assessment found Idris was one of Osama bin Laden's international couriers in the 1990s and later became a top doctor at Al-Qaeda's al Farouq training camp in Afghanistan. There was no immediate information on the post-transfer medical care arrangements for Idris, who suffers from auditory and visual hallucinations.

The Cuba-based facility was set up to house foreigners rounded up in counter-terrorism operations in the wake of the September 2001 attacks on New York and Washington, which killed nearly 3,000 people.

President Barack Obama had promised to shutter Guantanamo during his 2008 presidential campaign, saying it had damaged the reputation of the United States abroad. But Obama has been unable to do so since taking office, in part because of resistance from Congress.

Detainees have complained of mistreatment, and many were held for years without trial while others faced special military tribunals known as military commissions. The transfers follow the repatriation of two prisoners to Saudi Arabia and two more to Algeria earlier this month.

Amnesty International's Zeke Johnson praised the release but said the progress was not fast enough, calling on President Obama to do more.

"His administration has the authority under current law to do it and half the people still held are cleared to leave," said Johnson, director of Amnesty’s security and human rights program, who is at Guantanamo this week to observe pretrial motion proceedings for the five men accused of plotting the 9/11 attacks.

"He should keep the momentum going by turning this week's trickle of transfers into a torrent," Johnson added.

U.S. Lawmakers are working on measures to ease restrictions on sending detainees home or to third countries, but plans to close the facility have been thwarted by a ban on transferring them to U.S. soil and some allies' opposition to taking them in.

Al Jazeera and wire services