Shiite Houthi rebels sweeping across Yemen clashed with armed Sunni tribesmen for a second straight day on Saturday in violence that left at least 36 people dead in one province over two days and 26 killed in another, medics said.
Twelve rebels were killed in a rocket-propelled grenade attack on their vehicle, while four tribesmen died in fighting in Ibb, medics and local officials said.
The rebels have met increasing fierce resistance since they advanced into mainly Sunni Ibb province, southwest of the capital, on Wednesday.
Twelve rebels and eight tribesmen died in fighting around Ibb city on Friday.
The fighting came after hundreds of armed tribesmen demonstrated outside the governor's office in the city on Thursday evening demanding the withdrawal of the rebels.
Ibb governor Yehya al-Iryani urged "armed groups from all sides to leave the province and end violence," in a statement on the official Saba news agency.
He warned that if armed groups remained in Ibb, authorities "will, under the orders of President Abdrabuh Mansur Hadi... take all necessary measures to restore security and stability."
Houthis have clashed repeatedly in the province since Tuesday with Al-Qaeda militants, who have vowed to resist the Shiite rebel advance.
Authorities in Yemen have repeatedly accused Tehran of backing the Shiite rebellion.
Yemen, which has been in political deadlock since the rebels took control of Sanaa on September 21.
The rebels, who were previously based in the northern highlands where Yemen's Zaidi Shiite minority is concentrated, have since made significant advances in provinces south of Sanaa.
They took the Sunni majority Red Sea port city of Hudeida on Monday, and on Wednesday advanced on to Zaidi-populated Dhamar, as well as mainly Sunni Ibb.
Meanwhile, at least 26 people were killed on Friday, mostly Shiite Houthi rebels, in separate battles with Al-Qaeda militants and armed tribesmen in central Yemen, medical sources and eyewitnesses have said.
Sixteen Houthis were killed in clashes with Al-Qaeda militants in the Radaa area of Yemen's al-Bayda province, a medical source told Anadolu Agency.
The Houthi rebels set up armed protest camps in Sanaa in August, calling for the government to step down and demanding more power in state institutions.
Late on Thursday, they began dismantling their protest camps in Sanaa.
Protest organisers said in a statement the move was aimed at implementing the terms of a UN-brokered ceasefire agreement, calling on Houthis to withdraw from Sanaa and disarm after the appointment of a new prime minister.
On Monday, Hadi named Yemen's envoy to the United Nations, Khalid Bahah, as his new premier, a nomination the rebels appeared to approve.
Hadi on Friday discussed the situation in his country with US President Barack Obama on the telephone, Saba reported.
Sanaa residents said the Houthi rebels have widened their deployment across the capital since Thursday, erecting new checkpoints and seizing vehicles without number plates, for fear of attacks.