Shiite and Sunni Muslims fighters agreed to a ceasefire on Monday after days of clashes in a northern Yemeni town in which at least 100 combatants and civilians were killed.
Overnight shelling had added to the death toll, a Sunni Salafist spokesman said.
A Red Cross delegation had managed to enter the town of Damaj to treat and evacuate those wounded in the fighting, but a translator with the team was shot and killed, he said.
The ceasefire was announced by the U.N. envoy to Yemen, Jamal Benomar, according to the state news agency.
“We hope that there will be a permanent cease fire and that the problem will be solved for good,” Benomar said.
Sectarian rivalry between Sunni Salafists and Shiite Houthis in Damaj has cast a shadow over reconciliation efforts in Yemen, a neighbor of oil exporter Saudi Arabia and home to one of al Qaeda's most active wings.
The continued turmoil on several fronts in the country has alarmed Washington, which has stepped up aid and military support for the government as part of its global fight against Islamist militants.
The clashes in Damaj broke out on Wednesday when Houthi fighters, who control much of the border province of Saada, accused their Salafi rivals of recruiting thousands of foreign fighters to attack them.
The Salafis say the foreigners are students who travel from abroad to study Islamic theology at Dar al-Hadith academy.
“There were massive Houthi attacks throughout the night against Dar al-Hadith academy and student dormitories,” said Surour al-Wadi’i, a Sunni Salafi spokesman. “The death toll has risen to at least 100.”
The figure cited was for Salafis killed in the clashes. The Houthis have issued no figures for casualties on their side.
Wadi’i also said a Red Cross delegation managed to enter Damaj on Monday in a convoy of four vehicles, only to come under fire from Houthi snipers who shot and killed a local translator with the delegation.
The ICRC could not immediately be reached for comment.
The fighting has caused concern among world powers that helped ease long-ruling President Ali Abdullah Saleh from power in 2011 after months of protests. The ambassadors of the five permanent members of the U.N. Security Council and the Gulf Cooperation Council called for an end to the fighting.
“The ambassadors notice the amassing of troops outside the area which will exacerbate tensions and risk more violence,” they said in a joint statement.
Benomar said President Abd-Rabbu Mansour Hadi had sent a plane to evacuate seriously wounded people from Damaj to Sanaa.
On Sunday, Yehia Abuesbaa, head of a presidential committee trying to end the fighting, said Houthi fighters had reneged on a previous promise to stop attacks after a prisoner release.
The Houthis blockaded Damaj for weeks last year, accusing the Salafis of stockpiling weapons, a charge they deny.
Saada province is the base for a long-running Houthi rebellion against the government. Complaining of social, religious and economic discrimination in Yemen, the Houthis fought several battles with government forces between 2004 and 2010, when a truce was announced.