Around 300 suspects are being held at Camp X-Ray

The Justice Department says it has found a birth certificate in the southern state of Louisiana that appears to match his claim that he was born there.

Children born in the United States are automatically American citizens.

If the claims prove true, Mr Hamdi would be eligible to be transferred to the US mainland.

So far, only one US citizen, John Walker Lindh, has been found among Taleban and al-Qaeda fighters captured during the US-led campaign against terror.

Mr Hamdi is one of around 300 suspects being held at Camp X-Ray, in the US base at Guantanamo Bay.

Civilian trial

Mr Hamdi says he was born in Baton Rouge, Louisiana, where his parents were working.

He accompanied his parents to Saudi Arabia when he was a toddler.

The executive order, signed by President Bush last year allowing suspected terrorists to be tried in military tribunals does not extend to US citizens.

That means that Mr Hamdi could be eligible to be moved to the mainland and face a civilian trial like that being used to prosecute Mr Walker.

Mr Walker faces 10 charges including conspiring to kill Americans and providing support to terrorists, including Osama Bin Laden's al-Qaeda.

On Wednesday, lawyers for Mr Walker said he was "tortured" into confessing and was mistreated by the FBI on his arrest.