The rebel group has made granting access to areas under their control contingent on a flurry of conditions that aid agencies reject, in part because it would give the Houthis greater sway over who receives aid, documents and interviews show.

The Houthis' obstruction has hindered several programs that feed the near-starving population and help those displaced by the nearly 6-year civil war, a senior U.N. official said, speaking on condition of anonymity to discuss the situation.

"Over 2 million beneficiaries ... are directly affected," the official said.

The Houthis have been pushing back against U.N. efforts to tighten monitoring of some $370 million a year that its agencies already give to government institutions controlled mostly by the rebel group, documents show. That money is supposed to pay salaries and other administration costs, but more than a third of the money spent last year wasn't audited, according to an internal document leaked to the AP.

The U.N. has largely kept quiet in public about the pressure, but behind the scenes the agency and international donors are digging in against the Houthi demands. The AP spoke to seven workers and officials from U.N. and independent agencies about the situation. All spoke on condition of anonymity for fear of reprisals. The AP also saw dozens of documents, including emails of aid officials.

In October, the U.N. humanitarian coordinator for Yemen, Lise Grande, sent a letter to the Houthi-appointed prime minister complaining about a long list of demands.

The "overwhelming majority" of them impede or delay delivery of aid and many violate humanitarian principles, she said in the letter, a copy of which was obtained by the AP.

For months, the Houthis demanded a 2% cut from the entire aid budget be given to them, a condition the U.N. and donors rejected. In an email to the AP, a spokesperson for the U.S. Agency for International Development said Houthi attempts "to implement a tax on humanitarian assistance are unacceptable and directly contradict international humanitarian principles." The United States donated $686 million to Yemen in 2019, according to USAID.

Last week, the Houthis appeared to back off the 2% demand, but continue to press for other concessions, according to aid officials.

During a meeting in Brussels last Thursday, aid agencies and international donors threatened to reduce aid if Houthis continue to impose restrictions on U.N. operations in Yemen.

The situation "has reached a breaking point," they said in a statement.

At least one agency, the World Food Program, is currently considering cutting back the monthly food aid it delivers to 12 million Yemenis every other month, a U.N. official said. "It's unfortunate that people will suffer but this is on the Houthis," the official said. "They can't use people as hostages for too long."

The Houthis' demands have stoked longtime concerns among aid agencies over the rebels' diverting of humanitarian funds and supplies into their own or their supporters' pockets or toward their war effort.

Delivering aid ina war zone has always poseda problem for U.N agencies. But officials said the situation in Yemen has been especially challenging.

The Houthi have withheld visas and permissions for equipment and supplies and refused to grant clearances for U.N. missions to move through rebel-controlled areas. Aid workers said agency leaders' past willingness to concede to some of the rebels' demands has emboldened the Houthi leaders to push for more.

Nearly 300,000 pregnant and nursing mothers and children under age 5 haven't received nutrition supplements for more than six months because the Houthis "held beneficiaries hostage to the 2%" demand, another U.N. official said.

In another example, Houthi authorities for months delayed permission to distribute 2,000 tons of food — enough to feed 160,000 people — in the district of Aslam, where the AP previously found starving villagers reduced to eating boiled leaves. When approval came in November, the food had spoiled "beyond the point of salvage," another aid official said.

Houthi leaders have remained defiant in the face of U.N. pushback.

"Yemen will survive" if agencies suspend aid, Abdul-Mohsen Tawoos, secretary-general of the Houthi agency coordinating international aid, told European donors during a Jan 20 Skype call. Minutes of the call were obtained by the AP.

He said the Houthis wanted to reach an agreement with the U.N. and its donors, but "won't be bullied."

Tawoos accused Grande, the top U.N. official in Yemen, of sending false reports aboutHouthis restricting the movement of U.N. humanitarian operations. Houthi leaders have threatened to expel her from the country.

The U.N.'s massive aid program, totaling $8.35 billion dollars since 2015, is vital to keeping many Yemenis alive. The U.N. calls the situation in Yemen the world's worst humanitarian crisis.

Ten million people in the country are on the brink of famine and80% of the population of 29 million in 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.

The Iranian-backed Houthi rebels control the capital, Sanaa,and much of the country's north, where most of the population lives and the need for aid is greatest. They are at war with a U.S.-backed, Saudi-led coalition fighting on behalf of the internationally recognized government.

With the economy in freefall, the U.N. aid effort is a major source of foreign currency into the country.

Despite the disputes between the Houthis and theU.N., aid officials continue to appeal to international donors for money to address the crisis in Yemen.

Over the summer, Grande pleaded to donor countries for more funds to meet the $4.2 billion goal.

"When money doesn't come, people die," she said.

But one international aid official said more money isn't the issue.

"I don't want more funds. I want the space to spend what I have," he said.