SANAA, Yemen (AP) — Shiite rebels and their opponents battled across several provinces Monday after the start of a five-day humanitarian pause in the Saudi-led air campaign, Yemeni officials said.

Coalition planes carried out flyovers over Yemen but did not drop munitions, the officials said. Ground fighting erupted in multiple provinces within minutes of the start of the unilateral cease-fire late Sunday.

The rebels, known as Houthis, said in a statement that they fired missiles across the border at a Saudi military position in the kingdom's Jazan region. The Saudi-owned Al-Hadath news channel said Saudi forces "responded" to Houthi shelling in Jazan, without elaborating.

Fierce clashes also broke out in the nearby town of Sabr, which is on a key supply route. Officials aligned with pro-government fighters say they have regained control of the center of the town and trying to advance into northern neighborhoods. Witnesses who fled Sabr Monday morning say corpses of fighters lay in the street next to destroyed military vehicles.

Officials and witnesses said there were sporadic clashes in Yemen's central Marib province. In Taiz, mortar shells fired in the center of the city killed four civilians, security and medical officials said.

Anti-aircraft gunfire was heard in the rebel-held capital Sanaa as coalition planes buzzed overhead.

The humanitarian pause is intended to help allow humanitarian aid to be delivered to ease the suffering of civilians in the Arab world's poorest country.

All officials spoke on condition of anonymity because they were not authorized to speak to reporters.