The United Nations warned on Wednesday that 22 "life-saving" aid programs will be forced to close in Yemen in the next two months if countries do not pay more than one billion U.S. dollars in funding that they pledged earlier this year.

The United Nations describes the situation in Yemen, where the four-year-long war has killed tens of thousands of people and left millions on the brink of famine, as the world's worst humanitarian crisis.

In February countries pledged 2.6 billion U.S. dollars to help, but UN humanitarian coordinator for Yemen, Lise Grande, said less than half that had been paid. The UN said that of 34 key aid programs only three were funded for the year and 22 "life-saving" programs will need to close in the next two months.

"We are desperate for the funds that were promised. When money doesn't come, people die," Grande said in a statement. "It's heart-breaking to look a family in the eye and say we have no money to help."

UN aid chief Mark Lowcock called out Saudi Arabia last month and the United Arab Emirates on Thursday for only paying a "modest proportion" of the hundreds of millions of dollars they have pledged. Both countries each promised 750 million U.S. dollars.

Saudi Arabia leads a Western-backed military coalition that intervened in Yemen in 2015 in a bid to restore the government ousted from power by the Iran-aligned Houthi movement. The United Arab Emirates is a key member of the coalition.