Shiite rebels, who have been advancing into mainly Sunni areas of Yemen, met deadly resistance on Friday from Sunni tribesmen as well as Al-Qaeda militants, tribal and security sources said.
Heavily armed tribesmen battled the rebels around the city of Ibb, southwest of the capital, which the rebels entered earlier this week, the sources said.
Four rebels and two tribesmen were killed.
Explosions were heard across Ibb as rebels inside the city came under rocket-propelled grenade fire by 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.
Deputy governor Ali al-Zanam responded that the rebels had told him they advanced into Ibb to "confront what they described as security gaps and hunt down wanted" Al-Qaeda militants, local media reported.
The rebels clashed with Al-Qaeda militants overnight in Baida province further east, leaving "dozens" of casualties, tribal and security sources said.
The rebels have clashed repeatedly with Al-Qaeda in the province since Tuesday.
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The Sunni extremists have vowed to resist the Shiite rebel advance in the name of Sunni Islam.
Rival groups are seeking to exploit a power vacuum in impoverished Yemen, which has been in political deadlock since the rebels took control of the capital 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, they appeared to have taken control, unopposed, of Dhamar and Ibb provinces, security officials said.
The steady expansion of the rebels, known as Huthis, 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.
An unknown assailant on a motorbike hurled a grenade at a Huthi post in the capital on Wednesday, killing two people and wounding two others, rebels said.
And suspected Al-Qaeda militants executed a local Huthi chief, Khalil al-Riyami, who was apparently captured during Tuesday's clashes in Baida province.
The rebels' Zaidi sect, an offshoot of Shiite Islam, makes up approximately a third of the Sunni-majority country's population.