Yemen's army and tribal fighters killed nine suspected al Qaeda militants overnight in a southern city where central government control has been weakened by months of political paralysis, residents and local officials said.

Five militants, one of them a Saudi, were killed in shelling and clashes on the northern edge of Zinjibar, which the Yemeni government said it had liberated from Islamist fighters in September, an official said.

The Saudi, named Naif al-Qahtani, was a senior member of Ansar al-Sharia (Partisans of Islamic Law), a group linked to al Qaeda, said the official. He had travelled to Zinjibar from a neighbouring province to take part in the fighting.

Yemen's south has been sliding towards lawlessness and deprivation whilst political wrangling over the fate of veteran President Ali Abdullah Saleh goes on, unnerving oil-giant neighbour Saudi Arabia and Western powers.

Washington and Riyadh, both targets of foiled attacks by al Qaeda's regional Yemen-based wing, are worried that the upheaval and a growing security vacuum could give Islamist militants the space they need to thrive.

A local official said four militants were killed in another part of Zinjibar in clashes with the army and tribesmen who are fighting to take back towns and territory seized by Islamist fighters in recent months.

Despite regular government reports of gains against Islamist fighters in Yemen's south, attacks on security checkpoints and officials have continued.

Yemen's government says Ansar al-Sharia is the Yemeni wing of al Qaeda. Some analysts say other local militant Islamist groups could be at work.

(Reporting by Mohammed Mukhashaf; Writing by Isabel Coles; Editing by Peter Graff)