At least 55 fighters have been killed and more than 100 injured in battles in Yemen's Marib province, according to security officials.
The officials, who remain neutral in the conflict that has splintered the country, said most of the casualties were caused by air strikes and artillery.
They said fighters loyal to Yemen's government gained ground near the Marib dam and several other strategic areas on a key front in the fighting against Shiite Houthi rebels.
The pro-government fighters are advancing toward Sirwah, a main Houthi stronghold in the province. They are ultimately aiming for the rebel-controlled capital Sanaa, east of Marib.
The Houthi media centre said their forces repelled "mercenary attacks", killing dozens of opposing fighters.