The detainees at Guantanamo Bay are under heavy guard

Fifteen men captured by American troops during the war in Afghanistan will be released this week, the US authorities have announced.

The detainees have nothing further to tell the authorities about terrorist groups, the US defence department says.

It adds that the prisoners - held at Guantanamo Bay naval base on Cuba - have not been linked to any crime.

Human rights activists have criticised the conditions in which about 600 people are held without charge there, and the fact that they have no legal right to challenge their detention.

The US has come under particular criticism - including from the United Nations - for holding a small number of youths.

The BBC's Justin Webb in Washington says at least one of those to be released is under the age of 16.

Few have been released

The move was dismissed by the human-rights organisation Amnesty International as too little, too late.

"All of the prisoners held at Guantanamo should be charged or released," an Amnesty spokesman in the US said.

US Secretary of State Colin Powell wrote to the Pentagon last month, saying the detentions were undermining international support for the US war on terror.

Pentagon officials denied that this week's releases were connected to the complaint.

Our correspondent says it does not look as if this is the start of a wider operation to clear the camp.

In a few months' time, military tribunals are expected to begin hearing cases against those the US believes were involved in terrorism.

Enemy combatants

The US has refused to say exactly how many prisoners it holds, but there are believed to be about 660 from more than 40 different countries. Most are suspected of being members of the Taleban or al-Qaeda.

US Defence Secretary Donald Rumsfeld said at the weekend he would like to expedite the processing of prisoners held at Guantanamo Bay.

But Mr Rumsfeld said it was necessary to question the detainees thoroughly.

"It takes time to find out what intelligence they have," he told CNN.

"It also takes time to figure out what law enforcement process might be appropriate.

"What [Mr Powell] and I have been concerned about... is that it's taking so long," he said.

Hundreds of people were taken prisoner by US-led forces in the war launched to overthrow the Taleban after the 11 September 2001 attacks on New York and Washington, blamed on al-Qaeda.

The US started moving captives to its remote base in Cuba in January 2002. They have been classified as enemy combatants - not prisoners of war, who would have more rights.

None have been charged with a crime and few have been released.

Three children under the age of 16 are among the detainees, whose status means they have no rights to legal representation.