RIYADH, Saudi Arabia—A five-day cease-fire between a Saudi-led military coalition and Yemen’s Houthi rebels expired Sunday night, despite a plea for an extension from the United Nations’ envoy to Yemen.

The envoy, Ismail Ould Cheikh Ahmed, had called for at least a five-day extension at a conference in Riyadh earlier Sunday. The cease-fire expired with no official word from either side on whether it would be continued.

Saudi Arabia and its allies began the strikes in March, aiming to degrade the Houthi militants and restore the exiled president, Abed Rabbo Mansour Hadi. There were no reports of fresh strikes in the hour after it expired.

The expiration also raised the possibility of more fighting on the ground. Houthi militants have continued to clash sporadically with Saudi-supported forces during the cease-fire, including in the southern cities of Aden and Taiz.

Hussein Al Bukhaiti, a pro-Houthi activist, said if Saudi Arabia continued its bombardment of areas in northern Yemen near the Saudi border, the Houthis would intensify their retaliation.

“If the aggression increases, Ansar Allah will declare war on Saudi Arabia,” he said, using another name for the Houthis.

Saudi state television reported the expiration of the cease-fire, but Saudi officials couldn’t be reached for comment to confirm it.

Saudi Arabia and the Houthis had agreed to a cease-fire beginning Tuesday, giving humanitarian aid groups their first major opportunity to address shortages of food, fuel and medicine caused by the conflict.

Aid groups ran an increased number of supply deliveries during the window, mainly from Djibouti. They said Sunday that efforts had been largely successful, although difficulties stemmed from a fuel shortage and continued clashes in some areas.

“Some areas are still unacceptable where there is conflict on the ground,” said Qaseem Ghausy, a logistics coordinator in Yemen for the U.N. World Food Program.

International agencies including the WFP and the International Committee of the Red Cross mobilized thousands of tons of food, fuel and medicine during the pause.

The WFP brought in 120,000 liters of fuel and planned to supply rations for 750,000 people. The U.N. High Commissioner for Refugees was also airlifting in supplies, including blankets and sleeping mats from its warehouses in Dubai. A spokesman for the UNHCR said four of six planned flights had landed in Yemen as of Sunday afternoon, and the final two were en route.

The U.K.-based Islamic Relief Worldwide reached the vast majority of the 3,000 households it planned to distribute food aid to during the five-day pause, a spokeswoman said, and the remainder would be given out on Monday.

At the conference in Riyadh, the exiled Yemeni president called for international financial and political support for Yemen, while blaming the Houthis for plunging the country into conflict.

“They endangered local, regional and international security,” he said. “They aborted the peaceful transition of power that was agreed upon.”

Yemen’s crisis began last year, when Houthi militants marched toward the capital, San’a. They took control of it and Yemen’s government in February. In March, Mr. Hadi fled to Riyadh.

WSJ