WASHINGTON – Senator Dianne Feinstein, the California Democrat who is chairwoman of the Senate intelligence committee, declared on Thursday that it was time to consider lifting a ban on repatriating low-level detainees to Yemen from the prison at Guantánamo Bay, Cuba, amid rising desperation and a hunger strike among inmates there.

Of the 166 wartime detainees who remain at Guantánamo, 86 were approved for transfer more than three years ago, but remain in Cuba. Of those, 56 are from Yemen, where President Obama halted any further transfers after the attempted bombing of a Detroit-bound airliner by a Yemeni-based branch of Al Qaeda on Dec. 25, 2009.

In a letter to President Obama’s national security adviser, Tom Donilon, Senator Feinstein noted that at the time of that attack, she had urged Mr. Obama to adopt that transfer ban “until the situation in Yemen is stabilized.”

But now, she wrote, it was time to examine whether Yemen’s president, Abdu Rabbu Mansour Hadi, who has been a strong foe of Al Qaeda since taking office in 2012, could provide adequate security assurances to return “all 56 Yemenis previously recommended for transfer.”

Although Al Qaeda “still has a strong presence in Yemen, I believe it would be prudent to revisit the decision to halt transfers to Yemen and assess whether President Hadi’s government, with appropriate assistance, would be able to securely hold detainees in Sana,” she wrote.

The White House did not immediately respond to Ms. Feinstein’s letter, which was hailed by groups like Amnesty International and Human Rights First. She also urged the administration to revitalize its stalled efforts to close the prison, including appointing a successor to the ambassador who had negotiated transfers until he was reassigned and not replaced this year.

When he became president, Mr. Obama vowed to close the prison. But he also decided to keep holding without trial, as wartime prisoners, some detainees viewed as too dangerous to release but too hard to prosecute. Congress soon blocked his plan to bring them to a Supermax prison inside the United States.

Separately, in January 2011, after some former detainees engaged in terrorist activities after their release, Congress restricted transfers to countries with troubled security situations. Releases of lower-level detainees halted and did not resume even after lawmakers later gave the administration the authority to waive those restrictions on a case-by-case basis, a power it has not used.

In recent months the detainees have started protesting. The military said on Thursday that 94 prisoners were participating in a hunger strike. Ms. Feinstein said that monitors from the International Committee of the Red Cross recently told her staff that “the level of desperation among the detainees is ‘unprecedented’ in their view.”

“The fact that so many detainees have now been held at Guantánamo for over a decade and their belief that there is still no end in sight for them is a reason there is a growing problem of more and more detainees on a hunger strike,” she wrote.