A 24-hour ceasefire agreement between Yemen’s Shia Houthi rebels and Sunni tribal fighters has been reached in Ibb province, sources have told Al Jazeera.

At least eight people were killed in heavy fighting between tribesmen and Houthis on Friday in the central province, witnesses said, increasing fears of outright sectarian warfare.

Six Houthi fighters and two tribesmen were killed on the outskirts and inside the city of Ibb, 150km south of the capital, Sanaa.

Explosions were heard across the city as the Houthis came under rocket-propelled grenade fire from tribesmen in the surrounding countryside, witnesses said.

The city of Ibb borders Bayda province, a stronghold of al-Qaeda in the Arabian Peninsula (AQAP), where Houthis have clashed repeatedly with al-Qaeda-linked fighters since Tuesday.

Clashes overnight in Bayda left “dozens” of casualties, tribal and security sources said.

Rival groups are seeking to exploit a power vacuum in impoverished Yemen, which has been in political deadlock since the Houthi rebels took control of Sanaa on September 21.

The rebels, who were previously based in the northern highlands where Yemen’s Zaidi Shia minority is concentrated, have since made significant advances in provinces south of Sanaa.

Al Jazeera’s Omar Al Saleh, reporting from the capital, said Yemen is facing a new reality with the Houthis emerging as a powerful force.

“Many people here question the inaction of the military to stop the Houthi advance,” he said.

“Some believe a hidden alliance between former President Ali Abdullah Saleh, who still enjoys influence over military commanders, and the Houthis has been formed to settle old scores with their common enemies.”

The Houthis took the Sunni majority Red Sea port city of Hudaydah on Monday, and on Wednesday, they appeared to have taken control, unopposed, of Dhamar and much of Ibb provinces , security officials said.

The steady expansion of the rebels has increased the threat of an open confrontation with AQAP.

AQAP claimed responsibility for a powerful suicide bombing that killed 47 people at a gathering of Houthi supporters in Sanaa earlier this month. Al Jazeera