Nearly half of Yemen's 22 provinces are on the verge of famine as result of the war there and more than 13 million people need food aid, the UN says.
Aid groups have blamed curbs imposed by the Saudi-led coalition on access to Houthi-controlled ports for the crisis and also accuse Houthis of preventing supplies from reaching some areas, including the city of Taiz in the southwest.
"From a food security perspective, 10 of Yemen's 22 provinces are classified as emergency, which is one step before famine," Adham Musallam, deputy director of the UN World Food Programme office in the capital Sanaa, said on Wednesday as the agency launched a food voucher program to help the most needy.
Fighting over the past year has displaced about 2.3 million people and left more than half of Yemen's 26 million population in need of food aid, Mr Musallam said.
"This means that we must not wait until the situation reaches famine, but must act now to provide humanitarian aid directly," Mr Musallam said.
The Houthis took over Sanaa in September 2014, ousting President Abd-Rabbu Mansour Hadi, then seized his temporary headquarters in the southern port city of Aden.
The Saudi-led Arab coalition intervened in March 2015 to try to restore Hadi to power and roll back Houthi gains. More than 6200 people have been killed in the conflict, half of them civilians.