Shiite rebels sweeping across Yemen clashed with armed Sunni tribesmen for a second straight day on Saturday in fighting that left at least 16 people dead, medics said.
The rebels took control of the capital Sanaa on September 21 after orchestrating weeks of protests that paralysed the government. They then pushed south earlier this week, meeting little or no resistance from security forces.
But as their advance has taken them out of the mainly Shiite northern highlands into predominantly Sunni areas, they have met increasingly fierce resistance from local tribes as well as Sunni extremists of Al-Qaeda.
In mainly Sunni Ibb province on Saturday, 12 rebels were killed in a rocket-propelled grenade attack on their vehicle, while four tribesmen died in fighting, medics and local officials said.
It came after 14 rebels and 10 tribesmen died in fighting around Ibb city on Friday.
After those clashes, Ibb governor Yehya al-Iryani called on "armed groups from all sides to leave the province and end violence."
He warned that if they did not, authorities "will, under the orders of President Abdrabuh Mansur Hadi... take all necessary measures to restore security and stability."
But during the night, hundreds of armed tribesmen surrounded Ibb, laying siege to the rebels inside the city, witnesses said.
Saturday's clashes came as the rebels tried to send reinforcements from Shiite areas further north, tribal sources said.
Hadi discussed the situation with US President Barack Obama on Friday, but he has so far done nothing to stop the rebel advance.
The rebels took the Sunni majority Red Sea port city of Hudeida on Monday, and on Wednesday advanced into mainly Shiite Dhamar province and then Ibb.