CAIRO -- The regional chief of the UN children's agency said Saturday that Yemeni authorities are making it difficult to deliver much-needed humanitarian aid and warned that impeding relief efforts could plunge the country into famine.

Geert Cappelaere told The Associated Press in an interview from Yemen that recent U.S. calls for a cease-fire are imperative to ending the nearly four-year war, which pits a Saudi-led coalition against Iran-aligned rebels known as Houthis.

He visited the Red Sea port city of Hodeida and the rebel-held capital, Sanaa, over the past two days as clashes and airstrikes intensified. He said both the internationally-recognized Yemeni government and Houthi rebels "are not enabling us to do our work as fast as we should."

Cappelaere said he can't bring the best nutrition experts to the country because of delays in granting visas and that aid agencies face bureaucratic impediments the delay the import of supplies.

He criticized authorities with "other interests" for creating delays in the arrival and distribution of supplies, without elaborating.

Most aid agencies operate in Houthi-held areas where they face movement restrictions. The rebels manipulate aid distribution by providing lists of beneficiaries and sometimes divert aid to their supporters.

Cappelaere's visit came shortly after the United States called for the cease-fire within 30 days. He said the situation is deteriorating, with millions unable to meet their basic needs.

Yemen has been at war since March 2015, when Houthis occupied northern Yemen, forcing the government into exile. Since then, a Saudi-led coalition supporting the government has blockaded the rebel-held north and waged a devastating air campaign. The U.S. has sold billions of dollars' worth of arms to Saudi Arabia and provides logistical and other support to the coalition.

"An end to the conflict is ... a much-needed step but it needs to be complemented with investment and governance of this country that puts the interest of the people at the centre and the interest of the children at the core of politics," Cappelaere said.

Three-quarters of Yemen's 29 million people are food insecure, 1.8 million children suffer from malnutrition and 400,000 children under age 5 are at risk of death from starvation. Every 10 minutes a child dies of preventable diseases, according to UNICEF.

Around 40 per cent of the 400,000 are located in and around Hodeida, which has been the target of a stalled coalition offensive in recent months. The port is a key entry point for food and humanitarian aid, but the coalition says the Houthis also use it to import weapons.

During a visit to Hodeida's main hospital, Cappelaere said he saw children suffering from severe malnutrition and others paralyzed by complications from diphtheria, an epidemic fueled by the breakdown of health services. He says many families in Hodeida cannot make it to hospitals because of airstrikes and shelling, or because they cannot afford transportation.

"It's high time for authorities from both sides to take responsibility and enable that assistance without any conditions, without any hurdles," he said.