The Shiite fighters captured the strategic central city of Radmah in Ibb province on Wednesday, a city that links Sanaa with the main southern city of Aden, after prolonged fighting with local tribesmen, AFP reported.
Tribal sources said that nine fighters from both sides were killed during the battle for the city, known to be a stronghold of the Muslim Brotherhood-linked Islah party.
With the fall of Radmah, the Houthis now virtually control the whole of Ibb province with the exception of Udain, which is in the hands of al-Qaeda in the Arabian peninsula (AQAP).
The Houthis, who had long been concentrated in their northern highlands where Shiites form a majority, have been facing fierce resistance from local tribesmen as well as AQAP.
After easily overrunning the capital in September, the Houthis moved on to the Red Sea port city of Hodeidah as well as Shiite-populated Dhamar and entered Ibb, the provincial capital.
The Houthis appear unmoved by a call from President Abd-Rabbu Mansour Hadi to "immediately pull out" of all seized areas, including the capital.
The Houthis hail from the northern highlands and champion the interests of the Zaidi community, who make up a fifth of Yemen's 25 million population.