Troops loyal to former president Ali Abdullah Saleh on Thursday launched an attack backed by Saudi-led coalition forces against Yemen's Shia Houthi rebels, their former allies, military sources said.
They said the attack was launched under the command of Tareq Mohammed Abdullah Saleh, a nephew of the late president, in a region of southwest Yemen between the port city of Mokha and Taez province.
There were casualties on both sides, according to medical sources, but no figures were available.
Saleh's forces had been allied with the Huthis since the outset of Yemen's war in 2014, assisting in their capture of the capital Sanaa.
But the Houthis killed Saleh in December 2017 after accusing him of betrayal and holding secret talks with Riyadh, which heads an Arab coalition fighting the rebels to restore Yemen's internationally recognised government.
Nearly 10,000 people have been killed in Yemen since the coalition intervened in March 2015.
The Yemeni army has since splintered, with factions in Sanaa maintaining loyalty to the Houthis, other brigades allied with the coalition and some troops in southern Yemen standing by separatists.