By Jane Sutton

GUANTANAMO BAY U.S. NAVAL BASE, Cuba (Reuters) - U.S. Senator Dianne Feinstein urged the Pentagon on Wednesday to stop force-feeding hunger-striking prisoners at the Guantanamo Bay detention camp and called the practice "out of step" with medical ethics and international norms.

Feinstein, a California Democrat who chairs the Senate intelligence committee, sent a letter to Defense Secretary Chuck Hagel, saying the Guantanamo force-feeding policy was also out of synch with policies in the civilian federal prisons.

"Hunger strikes are a long-known form of non-violent protest aimed at bringing attention to a cause, rather than an attempt of suicide," she wrote.

"I believe that the current approach raises very important ethical questions and complicates the difficult situation regarding the continued indefinite detention at Guantanamo."

Speaking in Berlin on Wednesday, President Barack Obama said the United States would redouble efforts to close the prison at Guantanamo.

A Pentagon spokesman, Lieutenant Colonel Todd Breasseale, said he would not respond to Feinstein's letter to Hagel via the press, but added that "we will continue to treat the detainees in our charge humanely and that we will not allow them to harm themselves."

The U.S. military says 104 of the 166 prisoners have joined a four-month-old hunger strike to protest their indefinite detention at Guantanamo, where most have been held without charge for more than a decade.

Forty-four of those are being fed liquid meals through tubes inserted into their nostrils and down into their stomachs.

Feinstein, who visited the Guantanamo camp earlier this month with White House chief of staff Denis McDonough and Republican U.S. Senator John McCain, joins a growing chorus of critics who have called for an end to the force-feeding.

The International Committee of the Red Cross, the World Medical Association and the American Medical Association have said competent adults have the right to refuse treatment, including life-sustaining nourishment, and that force-feeding them is "never ethically acceptable."

Human rights groups have also said that force-feeding violates the Geneva Conventions' prohibition of cruel, humiliating and degrading treatment of captives held in armed conflict.

"I write to express to you my concerns and opposition to the force-feeding of detainees, not for reasons of medical necessity but as a matter of policy that stands in conflict with international norms," Feinstein wrote to Hagel.

She said an intelligence committee staff review had found significant differences between force-feeding policies at the Guantanamo camp, which holds foreign men captured in U.S. counter-terror operations, and the U.S. Bureau of Prisons, which holds people convicted of crimes.

Force-feeding is "extremely rare" in the Bureau of Prisons facilities, where no prisoner has been force-fed in more than six months, Feinstein said.

"When force-feedings do occur within the Bureau of Prisons, we have been told that nearly 95 percent of the time they are conducted with a fully compliant inmate requiring no restraints," she wrote.

"At Guantanamo Bay, on the other hand, all detainees being force-fed — regardless of their level of cooperation — are placed in chairs where they are forcibly restrained."

She said that in the Bureau of Prisons' facilities, force-feedings are videotaped, the warden must notify a sentencing judge and explain the reasons for the feedings, and the feeding procedures require an individual assessment of each inmate, "a practice that I found largely absent at Guantanamo Bay."

(Editing by Kevin Gray and Stacey Joyce)