USAID said late Monday that it will suspend aid to Houthi-controlled areas in Yemen, where most of the country's people live, if the rebels don’t remove impediments obstructing aid operations.

In a statement, the agency said it informed partners including U.N. agencies about the plan last week. It said the suspension will start in late March if Houthis take no action.

“We continue to do everything we can to avoid a reduction in aid in northern Yemen,” the agency said.

The U.S. provided about $700 million in aid to Yemen last year. It is among the largest donors to Yemen, where a U.N. aid program totaling $8.35 billion since 2015 is vital to keeping many Yemenis alive. The U.N. calls the situation in Yemen the world’s worst humanitarian crisis.

As the war in Yemen enters its sixth year, 10 million people in the country are on the brink of famine and 80% of the population of 29 million need of aid, according to the U.N. More than 3 million people have been displaced, cholera epidemics have killed hundreds, and at least 2.2 million children under 5 suffer from severe malnutrition, the agency said.

Yemen’s Houthi rebels, who control most of northern Yemen, have blocked half of the United Nations’ aid delivery programs in the war-torn country — a strong-arm tactic aimed at forcing the agencies to give them greater control over the humanitarian campaign.