Shiite rebels and gunmen from the powerful Hashid tribe in north Yemen clashed for a third straight day on Wednesday, with the fighting intensifying, tribal sources told AFP.
The fighting first broke out on Monday when Shiite Huthi rebels attempted to take over the towns of Wadi Khaywan and Usaimat, strongholds of the Hashid tribe in Amran province, they said.
The Shiites launched the attacks in retaliation for the Hashid tribe's support for hardline Sunni Salafist groups fighting Huthis in Dammaj, the Shiites' stronghold in the northern province of Saada, the sources said.
According to witnesses, the fighting has left dozens dead and wounded. AFP could not confirm the toll due to the difficulty of accessing the area.
The tribal sources said the fighting had intensified on Wednesday, while the Shiite Huthi Ansarullah (Partisans of God) group said on their website http://www.ansaruallah.com/ that they had taken control of several Hashid strongholds.
During the battles, a Hashid chief, Hashim al-Ahmar, escaped an attack but his guard and four of his relatives were killed, tribal sources said.
Yemeni President Abdrabuh Mansur Hadi on Tuesday sent a delegation to try to broker a truce but they have yet to make contact with leaders from the two sides.
Huthi rebels have been battling the Sanaa government for nearly a decade in the remote Saada province, but the outbreak of fighting with Sunni militants has deepened the sectarian dimension of the unrest.
Fighting that erupted in late October has centred for months on a Salafist mosque and Koranic school in Dammaj.
But the conflict has spread in the northern provinces, embroiling Sunni tribes wary of the power of the Huthis, who have repeatedly been accused of receiving support from Iran.
The Huthis, named after their late leader Abdel Malek al-Huthi, are part of the Zaidi Shiite community.
They rose up in 2004 against the government of ex-president Ali Abdullah Saleh, accusing it of marginalising them politically and economically.
They accuse radical Sunnis in Dammaj of turning the town centre into "a real barracks for thousands of armed foreigners", a reference to the Dar al-Hadith koranic school, where foreigners study.
On January 6, the International Committee of the Red Cross said it had evacuated 34 people wounded in the Dammaj clashes.
The ICRC said it has managed to enter Dammaj six times since the fighting resumed on October 24.