Yemeni security forces and supporters of a slain rebel preacher have been involved in a second day of deadly clashes that have claimed many lives.
In one incident, rebels killed three policemen in an ambush in Amran, a remote region north of Sanaa.
They are followers of Zaidi Muslim cleric Hussein al-Houthi, leader of the Believing Youth group, who was killed in September after weeks of fighting.
Security forces blamed the renewed clashes on his father, Badr al-Din.
Tribal sources quoted by AFP news agency say a total of 23 rebels and 10 government troops have been killed since the violence erupted early on Monday.
The trouble began in Saada province close to the border with Saudi Arabia before spreading to Amran province.
The official news agency reported that the elderly Badr al-Din al-Houthi had reneged on a ceasefire agreed with the state "and incited subversive elements to stir up strife in the Saada area".
Last week, he criticised the authorities for detaining hundreds of young rebels despite a government deal to free them following the ceasefire.
Troops sealed off villages in Saada province on the lookout for rebels.
The army used heavy artillery to shell rebel positions concealed in mountains, reports said.
The tribal sources said the clashes started when security forces tried to strip Mr Houthi's followers of heavy weapons.
The Zaidi sect is a moderate Shia Muslim group which dominates north-west Yemen.