Pentagon chief Chuck Hagel said it is "going to be very difficult" to close the controversial US prison at Guantanamo Bay by the end of President Barack Obama's term in two years.
In an interview with National Public Radio that aired Monday, Hagel also acknowledged that he had taken a cautious approach to approving the transfer of some detainees out of the prison, which had reportedly irritated some officials in the White House.
Obama has repeatedly vowed to close the prison for terror suspects at the US naval base in Guantanamo Bay, Cuba. But Congress has banned the inmates from being sent to the United States and put up other obstacles, and US diplomats have struggled to find countries willing to take them in.
The number of inmates has gradually declined as the Obama administration has tried to speed up the transfer of detainees to their home countries or other nations willing to accept them.
Asked if Obama will manage to fulfill his promise to shut down Guantanamo before leaving office, Hagel said: "It's going to be very difficult ... especially if the Congress further restricts where these last 122 detainees go."
"This isn't a simple, easy matter of 'Oh, let's just move 122 detainees,'" he said. "These people are there for a reason. And as you draw down into the last numbers there, these are the most difficult cases."
Hagel will be stepping down as defense secretary soon after having announced his resignation in November. Some White House officials had said he lost the confidence of the president and frustrated top aides by his refusal to move more quickly on the Guantanamo transfers.
The Pentagon chief has to formally certify each transfer that there is no imminent risk of the detainee returning to the battlefield, and Hagel said he took that responsibility seriously.
"Has there been a slowing of that (transfer process), which hasn't always made me popular in some quarters? Yes," Hagel said.
"I've made that very clear to the president and to everyone, to the Congress: If it's my responsibility by law, which it is as secretary of defense, then I will do everything I can because the American people rely on that."
Asked if he was confirming that he was under pressure from the White House to take swift action, Hagel said: "I'm not affirming anything ... I'm just saying not all people agreed with me."
When Obama entered the White House, there were more than 240 inmates at Guantanamo, now about half that number are left.
In his State of the Union speech last week, Obama renewed his pledge to close Guantanamo and the State Department has touted figures that purportedly show recidivism among released detainees has dropped sharply.