At least 15 soldiers were killed and four wounded by suspected al Qaeda militants in an attack on an army checkpoint in southeastern Yemen on Friday, an army official said.
The soldiers were ambushed as they were having lunch in a desert area near the city of Shibam, in the eastern province of Hadramout, residents said. The army official said the gunmen were likely to be al Qaeda militants.
State news agency Saba put the death toll at 18 soldiers in what it described as "a cowardly terrorist act" in Shibam.
Yemen, one of the poorest countries in the Arab world, is struggling to restore state authority after long-serving President Ali Abdullah Saleh was forced to step down in 2011.
Security in Yemen is closely watched by Western and Gulf Arab countries because of its proximity to top oil exporter Saudi Arabia and the presence there of an al Qaeda branch that has plotted bomb attacks against international airlines.
Hadramout, a centre of Yemen's modest oil production, has been hit by sporadic fighting between government forces and a big tribal confederation, after a senior tribesman was killed in a shootout at an army checkpoint in December.
Yemen's challenges also include rising sectarian tension in the north and a secessionist movement in the south.
At least 20 people have been killed in northern Yemen since Thursday after Sunni Muslim tribesmen fought with Shi'ite Houthi rebels in a bid to recapture territories seized earlier by the Houthis, tribal sources said on Friday.
The fighting in Yemen's Omran province is the latest in a series of deadly confrontations in the north between Shi'ite Houthis and their Sunni rivals, who include local tribes and ultra-conservative Salafi Islamists.