The fighting occurred in the turbulent Marib province on Tuesday, when the army launched an air and ground offensive against tribesmen suspected of damaging the 320-kilometer pipeline (199 miles), which carries oil to an export terminal on the Red Sea and produces around 180,000 barrels per day.

Yemen's oil and gas pipelines have repeatedly been sabotaged by Islamist fighters or tribesmen since the uprising in early 2011, resulting in fuel shortages and a drop in export earnings.

Army officers shot dead

In separate incidents, security officals say gunmen on motorbikes shot dead two senior Yemeni security officers in Sanaa on Tuesday.

They have been identified as intelligence officer General Fadl al-Zamani of the national security services, who was shot dead at Bab al-Yaman, the entrance to the old city in Sanaa; and Colonel Salim al-Gharbani, a member of the elite Republican Guard, shot in the Dar Solom district.

The shootings come days after the Yemeni President Abdrabuh Mansur Hadi announced the restructuring of the army and the defence ministry, to curb the influence of those linked to toppled strongman Ali Abdullah Saleh.

Hundreds of thousands of Yemenis rallied in support of the shake-up.

jr/mz (dpa, AFP, Reuters, AP)