Two days of fighting between Shiite rebels and gunmen from the powerful Hashid tribes in north Yemen have killed nearly 22 people, a tribal source said on Monday.
A presidential commission had on January 8 brokered a truce between the Huthi rebels and Hashid fighters, ending two days of clashes in Amran province.
The fighting first erupted when Huthis tried to seize the towns of Wadi Khaywan and Usaimat, both Hashid strongholds.
But battles resumed a week ago and intensified over the past two days, the tribal source said.
"There were 22 people killed on both sides during the last 48 hours," the source said, adding that fighting was concentrated on Usaimat, 140 kilometres (87 miles) north of Sanaa.
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The Huthis launched the attacks in retaliation for the Hashid tribe's support for hardline Sunni Salafist groups fighting Huthis in Dammaj, a rebel stronghold in the north.
The town in Saada province has been besieged by the rebels for months.
Huthi rebels have been battling the Sanaa government for nearly a decade in Saada, but the clashes with Sunni militants have deepened the sectarian dimension of the unrest.
Fighting that erupted in late October has centred on a Salafist mosque and Koranic school in Dammaj.
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.
Huthis, meanwhile, 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.
Yemeni troops began to deploy in Saada on January 11 to monitor a ceasefire between the Huthis and Salafists.