SANAA, Yemen — The Saudi-led coalition launched new airstrikes in at least two Yemeni provinces on Monday, piercing a humanitarian pause that started at midnight the previous day, security officials said.
The U.S.-backed coalition of mainly Gulf Arab countries has been waging an air campaign since March against the Iran-supported Shiite 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, without elaborating.
Fierce clashes also broke out Monday 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 were trying to advance into northern neighborhoods.
Witnesses who fled Sabr on Monday morning said that corpses of fighters lay in the street next to destroyed military vehicles. The witnesses spoke on condition of anonymity, fearing for their own safety.
There were sporadic clashes in Yemen's central Marib province, officials said. In the city of Taiz, mortar shells fired in the center of the city killed four civilians, security and medical officials said. All officials spoke on condition of anonymity because they were not authorized to speak to reporters.
Anti-aircraft gunfire was heard in the rebel-held Sanaa as coalition planes buzzed overhead.
The humanitarian pause was intended to help allow humanitarian aid to be delivered to ease the suffering of civilians in the Arab world's poorest country.
At the United Nations, U.N. 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 overall effects of the conflict have been "catastrophic," with the estimated 23,000 deaths and injuries across the country "a shocking testament to the suffering faced by the civilian population," Van Der Klaauw said. He called the damage to schools, mosques, hospitals, ports and other critical structures "unacceptable."
Van Der Klaauw once again pleaded for all sides to stop attacking civilians and destroying the infrastructure of the Arab world's poorest country. Relief efforts can only scale up with safe, unimpeded humanitarian access, he said.
"Current access constraints meant I had to reach Aden via Djibouti on a 12-hour boat tide," he said. "Humanitarians need more direct access."
The $1.6 billion humanitarian appeal for Yemen this year remains just 15 percent funded so far.
Associated Press writer Cara Anna contributed to this report from the United Nations.