Aid agency USAID has said it will suspend aid to Houthi-controlled areas in Yemen, where most of the country's people live, if the rebels do not remove impediments obstructing aid operations.

In a statement, the agency said it informed partners, including UN agencies, about the plan last week. It said the suspension will start in late March if Houthi rebels do not act to remedy the situation.

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

The US provided about $700m in aid to Yemen last year. It is among the largest donors to Yemen, where a UN aid programme totalling $8.35bn since 2015 is vital to keeping many Yemenis alive.

The Houthis took over the capital Sanaa in 2014 after overthrowing the government of Abd-Rabbu Mansour Hadi and establishing their authority in much of Yemen's northern region.

A Saudi Arabia-led coalition intervened in support of the Yemeni government in 2015 when Hadi fled into Saudi exile as the rebels closed in on his last remaining territory in and around Aden.

Worst humanitarian crisis

The war in Yemen has killed tens of thousands of people, most of them civilians, according to relief organisations. The devastating conflict has left millions facing the threat of famine in what the UN calls the worst humanitarian crisis in the world.

As the war enters its sixth year, 10 million people in the country are on the brink of famine and 80 percent of the population of 29 million is in need of aid, according to the UN.

More than three million people have been displaced, cholera epidemics have killed hundreds, and at least 2.2 million children under five suffer from severe malnutrition, the agency said.

Human rights groups have criticised the Saudi-UAE-led coalition for targeting civilians in hospitals, schools and markets, also condemning Western countries for providing it with arms.

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