(Updates death toll to nine)
SANAA, Aug 6 (Reuters) - At least nine Yemeni military personnel were killed on Tuesday when tribesmen shot down an army helicopter in central Yemen where gunmen had repeatedly blown up oil pipelines, a military source said.
The government has been frustrated by repeated attacks on Yemen's main oil export pipeline, often carried out by disgruntled tribesmen seeking personal gain or trying to force authorities to release jailed relatives.
A Yemeni military source said a brigadier general, who had commanded a military brigade, six of his military escorts and two crewmen from the military helicopter were killed when the aircraft came down while on a reconnaissance flight in the central Maarib province.
Government sources had earlier put the death toll at six.
Witnesses said the helicopter was engaging armed tribesmen when it was shot down.
"The helicopter was firing from a low altitude at armed tribesmen accused of blowing up the oil pipeline," one witness said. "The gunmen fired back with a machinegun and it crashed."
Defence Ministry officials could not immediately be reached for comment.
One of the most impoverished countries in the Arab world, Yemen faces a host of challenges as it tries to restore state authority nearly two years after long-serving President Ali Abdullah Saleh was forced to step down in 2011.
The country of 25 million depends on oil revenues for between 60 and 70 percent of its state budget. The last pipeline attack, on Saturday, came less than a week after it was repaired following a similar bombing. (Reporting by Mohammed Ghobari; Writing by Sami Aboudi; Editing by Jon Boyle and Sonya Hepinstall)