Fighting between Huthi rebels, members of the Shiite Islamic sect, and Hashid fighters began a week ago, Middle East Online reported.
A presidential commission had mediated a cease-fire between the groups Jan. 8.
The truce fell apart when Huthis tried to seize the Hashid towns of Wadi Khaywan and Usaimat in Amran province, officials said.
Fighting, which intensified in the past two days, was centered in Usaimat, 87 miles north of Sanaa, the capital.
The attacks were in retaliation for Hashid support of Sunni Salafists fighting Huthis in Dammaj, a town in Saada province that the rebels have besieged for months.
Yemeni troops began to deploy to Saada Jan. 11 to monitor the cease-fire.
The Huthis have been accused of receiving support from Iran.