Shia Huthi rebels have overrun strongholds of powerful tribes in northern Yemen, witnesses said Sunday, in a major advance following weeks of combat that have left scores dead.

The rebels have been pushing out from their stronghold in the mountains of the far north to other areas nearer the capital to expand their hoped-for autonomous unit in a promised federal Yemen.

The Huthis seized the town of Huth and Khamri village -- the seat of the Hashid tribal chief, as tribal defence lines crumbled, local sources and witnesses said.

The Huthis "completely took over the regions of Huth and Khamri," rebel spokesman Mohammed Abdulsalam told AFP by telephone.

Tribal chief sheikh Hussein al-Ahmar ordered his fighters to evacuate his family's farm in Khamri and set it ablaze, witnesses said, adding that the tribesmen have retreated to neighbouring areas.

Scores have been captured by the advancing rebels, they added.

The violence in Amran province dates back to January 5, and dozens of people have been killed in the fighting, including 60 on Friday alone.

Hashid tribesmen on Friday recaptured areas lost to the rebels earlier in the week, but the battle lines have shifted back and forth, according to local sources.

The capture of Khamri represents a severe blow to the powerful Ahmar clan, which leads the Hashid.

Divisions within the Hashid tribe could be behind the defeat, sources said, pointing out that some have sided with the Huthis.

The division is a result of an ongoing dispute between the Hashid chief sheikh Sadeq al-Ahmar and former president Ali Abdullah Saleh, who also belongs to the vast Hashid tribe.

Ahmar had sided with nationwide Arab Spring protests that forced Saleh to step down in February 2012 after 33 years in power.

In addition to taking on the northern tribes, the Shia Huthis have also been battling hardline Sunni Salafists who established religious schools in parts of the north.

The Huthis have accused the Salafists of bringing foreign extremists into their region.

Mediators dispatched by President Abdrabuh Mansur Hadi have succeeded in brokering ceasefires in several areas, but some of the truces have broken down.

The Huthis, named after their late leader Abdel Malek al-Huthi, are part of the Zaidi Shia community.

They rose up in 2004 against Saleh's government, accusing it of marginalising them politically and economically.

Hadi has pledged that Yemen will adopt a federal constitution in a bid to address local grievances that have fuelled violence across the Arab world's poorest country.

But at a ceremony last month to mark the conclusion of a troubled 10-month national dialogue, he put off any decision on the thorny issue of how many components it will have, promising that a special commission will decide.

The prospect of a federal Yemen, originally mooted as way to address grievances of the formerly independent south -- where secessionist violence has been on the rise -- has spawned demands for autonomy elsewhere, including the rebel-held far north.