Three United Nations agencies launched Thursday an "urgent appeal" for a Saudi-led military coalition to fully lift its blockade on Yemen, warning that millions would die if humanitarian aid cannot get through.

"We issue another urgent appeal for the coalition to permit entry of lifesaving supplies to Yemen in response to what is now the worst humanitarian crisis in the world," the heads of the World Food Programme, UNICEF, and World Health Organization said in a joint statement.

Earlier this week, the Saudi-led alliance eased its blockade, reopening some ports and airports, but the UN representatives said it was not enough.

"The space and access we need to deliver humanitarian assistance is being choked off, threatening the lives of millions of vulnerable children and families," they said.

Some 17 million people in Yemen had no reliable access to food, 14.8 million lacked basic healthcare, and almost 400,000 children are suffering from severe acute malnutrition, they said. There are also more than 900,000 suspected cases from a cholera outbreak.

In addition, "diphtheria is spreading fast with 120 clinically diagnosed cases and 14 deaths - mostly children - in the last weeks," and "at least one million children" are at risk of contagion if deliveries of vaccines and medicines continue to be blocked.

Yemen, one of the Arab world's poorest countries, has been locked in a devastating power struggle between a Saudi-backed government and Iran-supported Houthi rebels since late 2014, when the Houthis took over the capital Sana'a.