Houthi rebels have seized more territory south of Yemen's capital, including the city of Ibb, near an al-Qaeda stronghold.

Witnesses said that dozens of cars carrying Houthi fighters were seen arriving in Ibb on Wednesday, an area bordering al-Bayda province, a bastion of al-Qaeda in the Arabian Peninsula (AQAP).

The Houthis, who took over Sanaa on September 21, and who have held the strategic southern port of Hudaydah since Tuesday, did not face any opposition from local authorities as they entered the provinces of Dhamar and Ibb and set up checkpoints, officials said.

Residents said some Houthi fighters had gathered at the main stadium in Ibb, 150km south of Sanaa.

Another convoy of several cars carrying Houthis was later seen on the outskirts of Taiz, a city 50km south of Ibb.

The Houthis, who hail from the northern highlands and champion the interests of the Zaidi community which make up a fifth of Yemen's 25 million population, are increasingly imposing their authority outside the capital as well as in it.

The group wants the northern part of the country to be one region instead of three and are also seeking a bigger say in drafting the constitution.

Radda clashes

The Houthis seized Ibb hours after they had clashed in Radda with fighters from Ansar al-Sharia, an al-Qaeda-linked group.

Five Houthis, six suspected al-Qaeda fighters and a civilian were killed during the fighting in Radda, a security official and tribal sources said on Wednesday. The clashes forced dozens of families to flee.

Later on Wednesday, suspected al-Qaeda fighters took over the town of Odein, near Ibb, and clashed with security forces and Houthi fighters, residents and medical sources said.

AQAP, who view Shias as heretics and Houthis as being under the influence of Iran, last week claimed responsibility for a suicide bomb attack on a Houthi gathering in Sanaa that killed at least 47 people.

That attack was seen as a sign of AQAP's anger at the Houthis' takeover of Sanaa, a lightning assault that saw the group impose itself on the troubled administration of President Abd-Rabbu Mansour Hadi.

The Houthis ascent is the latest blow to central authorities in Yemen, who have struggled to keep control since mass protests in 2011 forced the country's long-serving president, Ali Abdullah Saleh, to step down.

In a separate sign of the fragility of Yemen's embattled state, southern separatists seeking to split from the north set an ultimatum for the government to evacuate its soldiers and civil servants by November 30.