(Updates death toll, government sources)
SANAA, Aug 6 (Reuters) - At least six Yemeni troops were killed on Tuesday when tribesmen shot down a military helicopter in central Yemen where gunmen had repeatedly blown up oil pipelines, government sources 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.
Government sources said a brigade commander, his security escorts and the helicopter crew died when the aircraft came down while on a reconnaissance flight in the central Maarib province.
Witnesses said the helicopter was engaging armed tribesmen when it was shot down. They said the pilot died in the attack.
"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)