The first civilian trial of a former Guantanamo prisoner has ended with the acquittal of a Tanzanian on all but one charge in the 1998 bombings of two US embassies.

Ahmed Khalfan Ghailani could still face life in prison for conspiracy to destroy government property, but a federal jury cleared him of all other 285 charges, including murder and conspiracy to commit murder.

Sentencing has been set for 25 January.

'We respect the jury's verdict and are pleased that Ahmed Ghailani now faces a minimum of 20 years in prison and a potential life sentence for his role in the embassy bombings,' a spokesman for the Justice Department said.

Critics have branded it a failure of the Obama administration's plans for civilian trials of Guantanamo prisoners - including Khalid Sheikh Mohammed, the self-proclaimed mastermind of the 11 September 2001 attacks.

After five days of deliberations, a federal jury in US District Court in New York found Ghailani guilty of conspiracy to destroy US property.

He was cleared of 276 murder and attempted murder counts, along with five other conspiracy charges.

It was a rare defeat for the US Attorney's Office in New York, which has a near perfect record in prosecuting terrorism cases.

Several pieces of testimony against him were thrown out of court because they were obtained by the CIA in unofficial detention centres outside the US.

Ghailani was held in CIA custody after his July 2004 arrest in Pakistan, moved to Guantanamo Bay in late 2006, and transferred to New York in June 2009 to stand civilian trial.

The US government accused Ghailani of buying seven gas cylinders used in the bomb and the truck used to transport it.

Prosecutors said Ghailani flew to Pakistan along with senior al-Qaeda operatives on the day before the bombings, and that a blasting cap was found in a cupboard in his room.

Defence lawyers called Ghailani a naive boy who was tricked by al-Qaeda and they denied Ghailani ever took the flight to Karachi.