Shiite rebels seeking to expand their territory across Yemen have clashed with Sunni tribesmen and Al-Qaeda fighters in violence that has left dozens dead, officials said Saturday.
Yemen has been dogged by political instability since an Arab Spring-inspired uprising forced veteran strongman Ali Abdullah Saleh from power in 2012.
Rival armed groups, including both the Huthi Shiite rebels and Al-Qaeda, have sought to step into the power vacuum.
At least 40 people are reported to have been killed in the past two days as the Huthi rebels push south with the security force largely absent.
The Huthis have already overrun the capital Sanaa and the Red Sea port city of Hudeida as they seek greater political clout in the country, located next to oil kingpin Saudi Arabia and important shipping routes in the Gulf of Aden.
As their advance has taken them out of the mainly Shiite northern highlands further south into predominantly Sunni areas, they have met increasingly fierce resistance from local tribes as well as Al-Qaeda.
The rebels lost 12 fighters in a rocket-propelled grenade attack on their vehicle on Saturday in Ibb province, medics and local officials said.
Four tribesmen were also killed in fighting.
In Rada further east, the rebels withdrew just hours after entering the town, following twin suicide bombings and rocket-propelled grenade fire by Al-Qaeda, tribal sources said.
The Huthi rebels took control of the capital Sanaa on September 21 after orchestrating weeks of protests that paralysed the government.
The fighting in Ibb 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.
Meanwhile, Ibb police chief General Fuad al-Attab resigned "to prevent further bloodshed and block the way for those seeking sedition and destruction" of the province, he said in a resignation letter seen by AFP.
The rebels had been demanding the resignation of Attab, a member of the powerful Muslim Brotherhood-linked Al-Islah (reform) Party.
President Abdrabuh Mansur Hadi discussed the situation with US President Barack Obama by telephone on Friday, according to Yemen's official Saba news agency.
Yemen is a key US ally in the fight against Al-Qaeda.
Washington regards the extremist group's Yemen branch as its most dangerous, and has carried out a drone war against its militants with the support of Hadi's government.
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.
In the southern province of Lahij, Al-Qaeda suspects opened fire Saturday at a vehicle carrying a local chief of pro-army militiamen who have fought extremists alongside troops, security officials said.
In Sanaa, meanwhile, dozens of Huthis gathered outside Saudi Arabia's embassy to protest a death sentence given by a Saudi court this week to prominent Shiite cleric Nimr al-Nimr for sedition.
They chanted Iran's Islamic revolutionary slogan: "Death to America! Death to Israel!", witnesses said.
Nimr was a driving force behind demonstrations against the Sunni authorities that erupted in Saudi Arabia's oil-rich east in 2011.
Authorities in Yemen and Saudi Arabia accuse Shiite-dominated Iran of backing the Huthi rebels.