Yemeni security and medical officials say mortar rounds fired by Shiite Houthi rebels have killed six civilians in the central city of Taiz, the country's third largest.

The security officials, speaking on condition of anonymity because they are not authorized to brief reporters, say Thursday's shelling came as reinforcements arrived to help the rebels maintain their grip on the city.

The officials say rebel fighters retreating from the south, where pro-government forces backed by Saudi-led airstrikes have been advancing in recent weeks, have joined the battle for Taiz.

The officials also reported airstrikes and ground fighting near the rebel-held capital, Sanaa.

The fighting in Yemen pits the Houthis and allied troops against southern separatists, local and tribal militias, Sunni Islamic militants and loyalists of exiled President Abed Rabbo Mansour Hadi.