Yemen's Houthi rebels have launched a deadly attack on one of the provincial headquarters of their biggest political rivals, killing four guards.

The Houthis attacked the headquarters of the Islamist Islah Party in Ibb province on Saturday, killing the guards in overnight clashes.

The Shia rebels reportedly detained 25 of the party's supporters after surrounding the building, taking them to an office of one of their leaders, despite calls from mediators trying to stop the fighting that those inside be allowed safe passage to leave.

Live Box 2014922125212206854

The Houthis also blew up a nearby dispensary belonging to Islah, forcing hundreds of people to flee the area, officials said on condition of anonymity.

The rebel fighters have won a string of successes on the battlefield across the country and view Islah, Yemen's biggest Sunni party, as a threat to their gains.

The Houthis took the city of Ibb last month after easily overrunning Sanaa in September in a surprise offensive that also saw them capture the Red Sea port city of Hodeida, Shia-populated Dhamar  and a number of other key cities.

The rebels, who hail from the northern highlands and champion the interests of the Zaidi community, have been able to expand their territory largely unchallenged by government forces, with the only real resistance coming from Sunni tribes supported by al-Qaeda in the Arabian peninsula (AQAP).

Ultimatum to president

Both the Houthis and AQAP have taken advantage of the lack of stability since autocratic President Ali Abdullah Saleh was ousted in 2012 following a year-long uprising.

Explainer: What's happening in Yemen
The Houthis have refused to relinquish their grip on power, and on Friday issued an ultimatum to President Abd-Rabbu Mansour Hadi to form a new government within the next 10 days.

The Shia group held a rally attended by about 30,000 tribal leaders in Sanaa, where they delivered a communique warning Hadi that "all options are open" if he failed to form a government.

However, a member of the Houthis' political bureau, Ali Al-Qahoum, denied the allegations in an interview with Al Jazeera.

Elsewhere on Saturday, clashes took place in the restive province of Radaa as two Houthi fighters were killed and a number of others wounded when unknown gunmen attacked their checkpoint.

Radaa has a mix of Sunni-Shia towns but has been a stronghold of AQAP and has seen frequent clashes betwen al-Qaeda-linked fighters and the Houthis.

In Hodeida, clashes were still raging on Saturday evening as AQAP killed ten security personnel in the district of Jebal Ras.

Source: Al Jazeera and agencies