Shiite rebels sweeping across Yemen clashed with Sunni tribesmen on Friday in violence that left 20 people dead, officials said.
Twelve Shiite rebels and eight tribesmen died in a battle for control of the predominantly-Sunni provincial capital Ibb which the rebels overran earlier this week, local government officials told AFP.
Explosions were heard across the city as the rebels, known as Huthis, came under rocket-propelled grenade fire from tribesmen in the surrounding countryside, witnesses said.
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."
However, the rebels who have so far captured several provinces and cities across Yemen, including the capital, have been met with little or no resistance amid an almost complete absence of government forces.
Deputy governor Ali al-Zanam said the rebels had told him they advanced into Ibb to "confront what they described as security gaps", local media reported.
The rebels clashed with Al-Qaeda militants on Thursday night in Rada, in Baida province further east, leaving "dozens" of casualties, tribal and security sources said.
Huthis have clashed repeatedly in the province since Tuesday with Al-Qaeda militants, who have vowed to resist the Shiite rebel advance.
But tribal, security and military sources said the Huthi rebels were met with no further resistance from Al-Qaeda as they captured Rada on Friday and deployed their armoured vehicles across the city, home to a mix of Sunnis and Shiites.
The Huthis' vehicles carried banners reading Iran's Islamic revolutionary slogan: "Death to America! Death to Israel!", residents said.
Authorities in Yemen have repeatedly accused Tehran of backing the Shiite rebellion.
Rival groups are seeking to exploit a power vacuum in impoverished Yemen, which has been in political deadlock since the rebels took control of Sanaa on September 21.
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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 Ibb.
The steady expansion of the rebels has increased the threat of an open confrontation with Al-Qaeda.
Al-Qaeda claimed responsibility for a powerful suicide bombing that killed 47 people at a gathering of Huthi supporters in Sanaa earlier this month.
- 'Revolt not over' -
The Huthi 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 Huthis 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 Huthi rebels have widened their deployment across the capital since Thursday, erecting new checkpoints and seizing vehicles without number plates, for fear of attacks.
Meanwhile, in the main southern city of Aden, troops shot and wounded seven supporters of the separatist Southern Movement as they marched towards an army camp, medics and witnesses said.
The separatists have been camped in Aden to press demands for renewed independence.
The south was independent between the end of British colonial rule in 1967 and its union with the north in 1990.
A secession attempt four years later sparked a brief but bloody civil war that ended with northern forces occupying the region.