Yemen has been battling a wave of al-Qaeda attacks as well as a rebellion by a Shia sect in the north and secessionist sentiment in the south.
   
The men, 11 Yemenis, four Syrians and a Saudi of Yemeni origin, were also accused of mortar attacks on the US and Italian embassies and a foreign housing complex in Sanaa.

Al-Qaeda had claimed responsibility for the attacks as well as an attempted attack on an oil refinery.

Neighbouring Saudi Arabia, the world's largest oil exporter, has said it fears instability in Yemen could allow it to become a launch pad for a revival of a 2003-2006 campaign by al-Qaeda that could destabilise the US-allied ruling Al Saud family.