The Yemeni commander of a vast military district covering half the country's border with Saudi Arabia pledged support on Sunday to exiled President Abd-Rabbu Mansour Hadi, local officials said.
The announcement puts at least 15,000 troops in the desert and mountain border area on the same side as Saudi Arabia, which backs Hadi and has waged an inconclusive three-week bombing campaign against Houthi rebels in Yemen who are allied with Iran.
"Brigadier General Abdulrahman al-Halily of the First Military District announced today his support for constitutional legitimacy as represented by President Hadi," one of the officials told Reuters.
The declaration was also broadcast on official radio in the city of Seiyun, the main city of the Hadramawt valley area where the district's main military base is located.
Most of Yemen's military is loyal to powerful ex-president Ali Abdullah Saleh, whose forces are fighting alongside the Shi'ite Muslim Houthi militia in battles stretching across Yemen's south and east.
But the defection of the northeastern troops brings to about 10 the number of divisions that back Hadi. It may point to an increasing sense in the military that momentum favors the president, who resides in exile in the Saudi capital Riyadh.
Beginning last week, most of the army divisions along Yemen's eastern Arabian Sea coast evacuated their posts and handed security of their bases and Yemen's Masila oil fields, the country's largest, to armed Sunni tribes.
Other powerful tribes followed suit on Monday within the First Military District, announcing after a vast gathering that they supported Hadi and the Saudi-led military operation, in a move which likely encouraged the commander's decision.
"The announcement followed that of the tribes. It would be impossible to oppose their will and continue to live side by side, and there was likely some pressure from Saudi Arabia, which is keen to secure its border," Radhi Subaih, a political analyst in Seiyun, told Reuters.
Citizens from Hadramawt, Yemen's largest province which stretches from the coast to the Saudi border, are linked closely to the kingdom by family and historical ties.
The remainder of Saudi Arabia's border with Yemen encompasses the northern stronghold of the Houthi guerrillas, who are avowedly hostile to Sunni-led Saudi Arabia.