A group of prisoners was transferred Thursday from the U.S. military prison for terrorists suspects in Cuba to Afghanistan, defense officials said.

The 15 prisoners left the naval base at Guantanamo Bay Thursday morning, two Defense Department officials said on condition of anonymity.

They gave no other details on their identities or nationalities.

It wasn't clear whether they were to be freed on arrival in Afghanistan or further detained for questioning or prosecution by that government.

That left some 645 prisoners from more than three dozen countries held at Guantanamo, without charges or access to lawyers. Officials refuse to identify them or their countries or say exactly how many there are.

The release comes as American officials try to more quickly sort out which terrorist suspects held in the high-security facility can be set free.

But it doesn't necessarily mean officials are getting closer to significantly reducing the prisoner population at Guantanamo.

Defense officials said earlier this week that as the group left, some 30 new prisoners would be brought to Cuba from Afghanistan -- an illustration that the global war on terror is a long-term undertaking.

They said the new detainees were captured in the continuing war against Al Qaeda in the Afghanistan region, as were most of the prisoners at Guantanamo.

But while the search for terrorists continues, officials of several government agencies have agreed to speed the processing of long-held suspects, two officials said.

Secretary of State Colin Powell in mid-April wrote a letter to Defense Secretary Donald H. Rumsfeld that gives a glimpse into administration discussions of the problem.

In what officials have said was a strongly worded letter, Powell cited complaints from allies in his argument that the indefinite holding of foreign citizens undermines efforts to win international cooperation in the war on terror.

Officials said the letter did not prompt the upcoming release, but that officials of several government agencies have been talking about the problem and agreed in recent weeks to try to expedite some releases.

Officials have long said that some prisoners could be released to their countries if it were certain their governments would deal with them properly. Talks have been under way with various nations, but no results have been announced.