Three boys aged between 13 and 15 are among about 660 inmates at the controversial camp, a US military official told the Guardian, on condition of anonymity. The official would not disclose their nationalities but said they had been brought from Afghanistan this year on suspicion of terrorism.

As soon as their ages were confirmed in medical tests, the children were moved to a "dedicated juvenile facility" at the camp, where they could socialise with each other, according to Lieutenant Colonel Barry Johnson, a spokesman at the base.

"They are in a secure environment free from the influences of older detainees," Lt Col Johnson said. "They are receiving specialist mental health care, in recognition of the difficult circumstances that child combatants go through, and some basic education in terms of reading and writing." Efforts were under way to contact their home nations, he added.

But the children would still be held indefinitely and would not be granted access to lawyers, he said, because the US continues to view them as "enemy combatants" - a term it has used to argue that the Geneva Conventions do not apply to the inmates, who have not been charged with any crimes.

That would be the case "until we ensure that they're no longer a threat to the United States, that there's no pending law enforcement against them, that they're no longer of intelligence value," Lt Col Johnson said.

Holding the children was "wholly repugnant and contrary to basic principles of human rights," said Angela Wright of Amnesty International, and contravened UN rules with "near-universal acceptance" regarding the treatment of juveniles.

The United States and Somalia are the only member states of the United Nations no to have ratified the Convention on the Rights of the Child, but the US is a signatory, and thus has "an obligation not to defeat the object and purpose of the treaty," Ms Wright said. "This is clearly totally at odds with the purpose of the treaty."

The precise legal ramifications are unclear, since many experts argue that the US is already in breach of international law by holding any of the detainees indefinitely without trial or charge, regardless of their ages.

Guantanamo Bay has attracted the condemnation of human rights campaigners since the first detainees arrived at what was Camp X-Ray, in January 2002. Soon after, they were pictured cowed, blindfolded and bound in the intense Cuban heat.

Since then, the US has built Camp Delta, a permanent and better-equipped facility, and has been at pains to describe how the inmates' religious and cultural preferences are being catered for. Representatives of the International Committee of the Red Cross are in regular contact with the inmates.

But reports of hunger strikes and attempted suicides have continued to emerge from the base. Military officials have confirmed 25 suicide attempts by 17 people since the inception of the camp, with 15 this year, often by inmates attempting to strangle themselves.

One detainee who reportedly fell into a coma after trying to hang himself was back off life support this week, Lt Col Johnson said, but there was no word on what the authorities would do with him next. The Pentagon has published regulations for how the inmates, who come from 42 countries, might be tried by military tribunals, but has not yet nominated any of them for trial.

The US court of appeals ruled last month that the government was entitled to deny due legal process to the detainees because they are not Americans and are not being held on US territory.

The three boys are not the only inmates under 16 to have been brought to Guantanamo Bay. Canadian officials have been seeking for months to gain access to Omar al-Khadr, a Canadian national who they say is being held at Camp Delta after being captured on July 27 during fighting in eastern Afghanistan. He was 15 at the time, they said.