Shiite rebels sweeping across Yemen met deadly resistance from both Sunni tribesmen and Al-Qaeda Saturday as they pushed south into mainly Sunni areas with the security forces largely absent.
Yemen has been dogged by political instability ever since an Arab Spring-inspired uprising forced veteran strongman Ali Abdullah Saleh from power in 2012.
Rival armed groups, including both the Shiite rebels and Al-Qaeda, have sought to step into the power vacuum.
In predominantly Sunni Ibb province, the rebels lost 12 fighters to an ambush in a second straight day of clashes with Sunni tribesmen, medics and local officials said.
In mixed Sunni-Shiite Rada further east, they withdrew just hours after entering the town, following twin suicide bombings and rocket-propelled grenade fire by Al-Qaeda, tribal sources 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 the Sunni extremists of Al-Qaeda.
The latest fighting in Ibb province, which also killed four tribesmen, came despite an ultimatum by its governor, Yehya al-Iryani, for "armed groups from all sides to leave the province and end violence."
Fighting for Ibb city killed 14 rebels and 10 tribesmen on Friday and Iryani threatened to 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.
President Abdrabuh Mansur 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, known as Huthis from the name of their leading family, took the Sunni majority Red Sea port city of Hudeida on Monday, and on Wednesday advanced into Dhamar province and then Ibb.
In neighbouring Baida province, the rebels briefly entered the town of Rada late on Friday, after Al-Qaeda militants pulled back following days of deadly fighting, tribal sources said.
But as the rebels entered in dozens of armoured vehicles, Al-Qaeda militants hit back with two suicide bombings and rocket-propelled grenade fire against a rally in the town centre, tribal sources said.
Al-Qaeda has vowed to fight the rebels in the name of Sunni Islam and claimed responsibility for a suicide bombing that killed 47 Huthi supporters in Sanaa earlier this month.
It has also carried out a string of other attacks against the rebels, who have vowed to hunt down the militants responsible.
Washington regards Al-Qaeda's Yemen branch as its most dangerous and has carried out a drone war against its militants with the support of Hadi's government.