Yemeni security officials say at least 55 fighters have been killed and more than 100 injured in battles in Marib province, a key front in the fighting against Shiite Houthi rebels.

The officials, who remain neutral in the conflict that has splintered the country, say most of the casualties were caused by airstrikes and artillery. They say fighters loyal to Yemen's government gained ground near the Marib dam and several other strategic areas.

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 center said their forces repelled "mercenary attacks," killing dozens of opposing fighters.

All officials spoke on condition of anonymity because they were not authorized to speak to reporters.