SANAA/ADEN: The latest Yemeni cease-fire unraveled Monday as new Saudi-led coalition airstrikes hit at least two provinces and clashes between Houthi rebels and loyalists broke out across the country.

The U.S.-backed coalition of mainly Gulf Arab countries has been waging an air campaign since March against the Iran-supported Houthi rebels, who control most of northern Yemen and the capital, Sanaa.

Two of Monday’s airstrikes killed 15 fighters allied with the coalition in the province of Lahj, security officials and field commanders said. More than 40 fighters were wounded in the apparently accidental strikes, they said, adding that the death toll was expected to rise.

The strikes happened near the strategic military base of Al-Anad, which is held by the rebels known as Houthis, and which was also hit by coalition planes Monday. The coalition also struck north of the port city of Aden.

Airstrikes had been halted for hours but ground fighting erupted in multiple provinces within minutes of the start of the unilateral cease-fire late Sunday.

The 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.

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 regained control of the center of the town and were trying to advance into northern neighborhoods.

Witnesses who fled Sabr Monday morning said that corpses of fighters lay in the street next to destroyed military vehicles. The witnesses spoke on condition of anonymity.

There were sporadic clashes in Yemen’s central Marib province, officials said. In the city of Taiz, mortar bombs fired in the center of the city killed four civilians, security and medical officials said. All officials spoke on condition of anonymity.

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

In a heavily bombed district of the capital, a 9-year-old girl, wearing a pink dress and holding her father’s hand, said: “Today I walked out to the corner shop to buy chocolate without fear ... We want this war to stop and [to] return to our school.”

Jamil, 40, owner of a car service shop in the Nahedeen district, said that a five-day cease-fire “doesn’t change anything ... but we and our children were able to sleep without air raids last night for the first time in a long time.”

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

At the United Nations, officials continued to express hope that the humanitarian pause could still work.

“We know the pause will have taken hold when we are able to deliver aid ... when Yemeni humanitarian workers themselves feel safe to get in a truck, drive that truck and deliver aid,” Stephane Dujarric, the spokesman for the U.N. secretary-general, told reporters Monday.

The past four months of fighting have devastated Aden and destroyed the lives of the majority of its people, the U.N.’s humanitarian coordinator for the country, Johannes Van Der Klaauw, said in a statement Monday, a day after visiting the city.

The United Nations says the conflict has killed more than 3,640 people, around half of them civilians, since late March.

A U.N.-declared six-day truce failed to take hold earlier this month after it was ignored by both the coalition and the rebels.

Relief supplies, however, have begun to trickle into Aden after loyalist fighters secured the city.

Several ships have docked in Aden since last Tuesday carrying thousands of tons of aid supplies sent by the U.N. World Food Program and Gulf nations.

But distributing the aid, particularly outside the city, poses a major challenge.