Security and tribal officials say fighting is underway in multiple Yemeni provinces on the fourth day of a humanitarian truce between a Saudi-led coalition and Shiite rebels.

The five-day truce has been repeatedly violated, with each side blaming the other.

The officials, speaking on condition of anonymity because they were not authorized to brief journalists, reported fierce fighting Saturday in the southern city of Aden, the western city of Taiz and the province of Marib.

They say three civilians were killed in Aden by mortar shells during an exchange of fire.

The conflict pits Shiite Houthi rebels and allied military units against forces loyal to embattled President Abed Rabbo Mansour Hadi, who fled the country in March. The Saudi-led coalition, which is supporting Hadi, began the airstrikes on March 26.