A US appeals court has partially overturned the Guantanamo conviction of an al-Qaeda publicist, saying a military commission lacked authority to convict on two of three charges.

The US Court of Appeals for the District of Columbia vacated the conviction of Yemeni prisoner Ali Hamza al Bahlul for providing material support for terrorism and solicitation of others to commit war crimes.

It upheld his conviction for conspiracy to commit war crimes.

"The Government offers little domestic precedent to support the notion that material support or a sufficiently analogous offense has historically been triable by a military commission," Judge Karen LeCraft Henderson wrote.

Bahlul acted as a publicist for al-Qaeda, making recruiting videos and taping the wills of some of the hijackers who slammed planes into the World Trade Centre, the Pentagon and a Pennsylvania field on 11 September, 2001.

Three months after the attacks, Bahlul was captured in Pakistan and transferred to the Guantanamo Bay US Naval Base in Cuba.

A military commission later convicted him of all three crimes and sentenced him to life in prison at the detention centre there.

The court also ruled that the solicitation charge against Bahlul "is plainly not an offence traditionally triable by military commission".

That charge was based on a violent video Bahlul made encouraging attacks on US targets.

"While we are still reviewing the decision, we are pleased that the Court rejected Bahlul's challenge to his conviction for conspiracy to commit war crimes," Marc Raimondi, a spokesman for the Department of Justice, said in an email.

Bahlul's attorney could not be reached for comment.