The humanitarian assistance to Yemen's northern areas could be reduced next month due to escalating restrictions imposed by the Houthi rebel group, a UN official said Wednesday.

"Threats, intimidation and restrictions against aid workers have been increasing in recent months in the Houthi-controlled areas," the UN official said on condition of anonymity.

"We face long delays in getting permits for assessing the needy people, distributing aid or treating food storage," he said.

"The other reasons were manipulation to the beneficiary lists and the (rebels') refusal to permit the using of biometric registration system for preventing aid fraud," he added.

Meanwhile, Abdulmohsen Tawoos, head of the Houthi-run humanitarian affairs coordination council, said in a statement on Sunday that "the measures taken by the authorities in Sanaa are primarily aimed at protecting the humanitarian aid organizations and their workers."

"We will make every effort to overcome all problems ... and provide all the facilities necessary for the success of the humanitarian organizations," Tawoos said in the statement, reported by the Houthi-controlled Saba news agency.

Yemen has been mired in a civil war since late 2014, when the Iran-backed Houthis seized control of much of the country's north and forced the Saudi-backed government of President Abd-Rabbu Mansour Hadi out of Sanaa.

The war has killed tens of thousands of people, displaced 3 million and pushed the country to the brink of famine.