SAN JUAN, Puerto Rico -- The United States announced the transfer Tuesday of six Guantanamo Bay prisoners back to their home countries, including one who, according to his lawyers, now may face abuse in Tunisia for nonviolent political activities.

With the transfer of four men to Yemen and two to Tunisia, the U.S. military says it now holds about 375 men at its base in southeast Cuba on suspicion of terrorism or links to al-Qaida or the Taliban.

The United States did not disclose the names of the prisoners, but lawyers and human rights groups identified one as Abdullah bin Omar, a 50-year-old Tunisian who has been held without charge since August 2002.

Attorney Zachary Katznelson of the British human rights group Reprieve said bin Omar faces "grave risk" of abuse and torture in Tunisia for his involvement with Ennahdaha, which he described as a moderate, nonviolent Islamic political party.

"I hope and pray Tunisia is going to do the right thing but I don't know that we can rely on that," Katznelson said in a phone interview from London. "We are truly, truly concerned for Mr. bin Omar."

He said Reprieve tried without success to persuade the United States to halt or delay bin Omar's transfer after his family said he had been convicted in absentia and sentenced to 23 years in prison for his involvement with a banned political group.

Katznelson said he had only been able to meet once with bin Omar, who may not have known he had been convicted.

"He said he had been told by Tunisian intelligence officers who visited [Guantanamo] that they had nothing on him. Clearly, that is not the case," the lawyer said.

Bin Omar, who is married and has eight children, fled Tunisia to avoid political persecution, according to Reprieve, and unsuccessfully sought political asylum in Pakistan, where he was living when he was captured by the United States.

A U.S. military spokesman, Navy Cmdr. Jeffrey Gordon, said no detainees are transferred out of Guantanamo without "credible assurances" from their government that they will be treated humanely.

Since Guantanamo opened in 2002, the U.S. has released about 405 prisoners after determining they were not a threat, did not have any intelligence value and their home country would be capable of preventing them from "rejoining the fight" against the United States or its allies.

About 80 of the remaining prisoners have been cleared for transfer or release and are awaiting this determination.

Many of those transferred to the custody of their native countries have been released.

Obtaining the necessary assurances from Yemen has been difficult, which makes Tuesday's announcement that four detainees were sent to that country relatively rare. There are about 100 Yemeni citizens in Guantanamo, more than from any other nation.

A group of U.S. defense attorneys traveled to Yemen last month to urge the government to lobby more aggressively for the release of their clients, following the lead of Western nations that have used diplomatic pressure to bring their nationals home.

Yemen's foreign minister, Abu Bakr al-Kerbi, later said at a news conference that it wants the detainees handed over to be "tried according to the constitution and the Yemeni laws."