I wouldn't make predictions about what we do with him

Pentagon spokeswoman Victoria Clarke

Officials are currently investigating Mr Hamdi's claim to US citizenship.

His birth certificate, which states he was born in Baton Rouge, Louisiana, appears to corroborate his claim.

"I think from the very beginning, there was a possibility in everyone's mind that he might be an American because he spoke English," said Army General Tommy Franks, who is in charge of the Afghanistan campaign.

"I can tell you that at the time he left Afghanistan we could neither confirm nor deny that he was an American citizen," he said.

Mr Hamdi will not immediately join the other American captured in Afghanistan - John Walker Lindh - who is now being held in a civilian jail outside Washington DC.

Mr Walker is being tried in a civilian court as US President George W Bush decreed that no US citizen should face the special military tribunals he authorised for suspects in the war against terrorism.

President Bush said no US citizen would face anti-terrorist military tribunals

Senior Pentagon officials cautioned that no decision had yet been taken on what to do with Mr Hamdi, including whether to bring any charges against him.

Pentagon spokeswoman Victoria Clarke said officials were fairly certain Mr Hamdi was a US citizen.

But she added: "I wouldn't make predictions about what we do with him."

Captured

Ms Clarke declined to say when officials started looking for the birth certificate or when it was found.

Mr Hamdi is reported to have been claiming for months that he was born in the US.

Both Mr Hamdi and Mr Walker were captured in December in an uprising in the northern Afghanistan city of Mazar-e-Sharif.

They were among prisoners from Afghanistan's former ruling Taleban and Osama Bin Laden's al-Qaeda network - accused of the 11 September attacks on New York and Washington.

Mr Hamdi was among 300 suspected Taleban and al-Qaeda fighters transferred from Afghanistan to the US military's Camp X-Ray in Guantanamo Bay, Cuba.