A woman waits to be registered before a food distribution carried out by the United Nations World Food Program in South Sudan on Feb. 26. (Siegfried Modola/Reuters)

President Trump has proposed large cuts to foreign aid at a time of acute need across Africa and the Middle East, with four countries approaching famine and 20 million people nearing starvation, according to the United Nations.

It is the first time in recent memory that so many large-scale hunger crises have occurred simultaneously, and ­humanitarian groups say they do not have the resources to respond effectively. The United Nations has requested $4.4 billion by March to “avert a ­catastrophe,” Secretary General António Guterres said last week. It has so far received only a tiny fraction of that request.

The details of Trump’s ­budget proposal have not been released, and large cuts to ­foreign assistance will face stiff opposition from Congress. So far, U.S. funding for the hunger crises has come out of a budget approved last year under President Barack Obama. But the ­famines or near-famines in parts of ­Somalia, South Sudan, Nigeria and Yemen underscore the reliance on continued U.S. assistance to save some of the world’s most desperate people.

In Nigeria, millions have been displaced and isolated by Boko Haram insurgents. In ­Somalia, a historic drought has left a huge portion of the ­country without access to ­regular food, as al-Shabab ­militants block the movement of humanitarian groups. In South Sudan, a three-year-old civil war has forced millions of people from their homes and farms. In Yemen, a civil war along with aerial attacks by a Saudi-led coalition have caused another sweeping hunger crisis.

In 2016, the United States contributed about 28 percent of the foreign aid in those four countries, according to the ­United Nations.

View Graphic Everything you ever wanted to know about the U.S. foreign assistance budget

“Nobody can replace the U.S. in terms of funding,” said Yves Daccord, the director general of the International Committee of the Red Cross (ICRC), who said of the current crises: “I don’t remember ever seeing such a mix of conflict, drought and extreme hunger.”

American aid officials said they were still trying to discern what the White House was planning to allocate to humanitarian assistance. Even though foreign aid is typically around 1 percent of the ­government’s budget, that is enough to make the United States by far the world’s largest donor. Last year, the United States contributed $6.4 billion in humanitarian aid, according to the United Nations, more than a quarter of global funding.

“We remain committed to a U.S. foreign policy that advances the security, prosperity and values of the American people,” said a spokesman for the U.S. Agency for International Development (USAID), who added that he was not authorized to speak on the record.

But asked whether the United States planned to contribute to the new U.N. appeal for hunger relief, the USAID official said, “We have no new funding to announce at this time.”

Early reports said Trump planned to propose 37 percent cuts to the State Department and USAID budgets. Many experts said they expected that those cuts would exclude U.S. contributions to security ­assistance.

“That leaves a much smaller component, which takes us directly to cuts in humanitarian assistance,” said Scott Morris, a senior fellow at the Center for Global Development.

The four hunger crises pose an enormous challenge for the humanitarian community, which is now torn among those emergencies. The last time a famine was declared in Africa was in Somalia in 2011. Nearly 260,000 people died, and aid groups later determined that they had waited too long to act. Famine is only declared when at least 30 percent of a population is acutely malnourished, and two adults or four children per every 10,000 people are dying each day.

Humanitarian groups have tried to apply the lessons from the 2011 disaster by moving quickly at the signs of deepening food crises. But the number of countries at risk of famine ­simultaneously makes a swift, thorough response to each of them very difficult.

“The donors are struggling left, right and center with their own allocations,” said Silke ­Pietzsch, the technical director for Action Against Hunger. “There are just too many fires to take care of.”

The United Nations was, by its own admission, late to recognize the scale of the crisis in ­northeastern Nigeria. Last year, when aid workers from Doctors Without Borders began traveling to parts of the country that had been blocked by Boko Haram fighters, they found soaring ­malnutrition rates and scores of people dying of preventable ­illnesses. Now, huge swaths of the region are still inaccessible to aid workers.

“No one can go 15 miles outside of the local government capitals,” said Yannick Pouchalan, the country director for Action Against Hunger. “There are still many people without any access to ­humanitarian assistance.”

USAID has been the largest provider of assistance in the crisis, Pouchalan said.

“If that aid stops, it means we won’t reach the people in need,” he said.

None of the crises are strictly about a lack of food aid or humanitarian funding.

“These are man-made crises in need of political solutions,” ­Pietzsch said.

In South Sudan, where two counties are already in the midst of famine, continued clashes ­between government and opposition forces have restricted the access of aid workers and kept people from farming on their land. The United Nations and other humanitarian groups have frequently been targeted by armed groups affiliated with both sides of the conflict. During fighting in July, government forces stole 4,500 metric tons of food from a World Food Program compound in Juba, the capital, enough to feed more than 200,000 people.

More than 1 million children in the country are malnourished and could die without a rapid intervention, according to ­UNICEF.

The United States has given more than $2.1 billion to South Sudan since the start of the conflict in December 2013. USAID claims that American food donations reach 1.3 million people every month and have “saved lives and helped to avert famine for three consecutive years,” according to a State Department statement last week.

Yet as the situation there ­worsens and food prices ­continue to rise as a result of an unusually bad harvest across much of Africa, the need for humanitarian assistance is expected to grow. In South Sudan, 700,000 people are already in “phase four” of the hunger crisis, the last stage before famine.

In Somalia, Save the Children has warned that the country has reached a “tipping point” and could quickly enter a famine “far worse than the 2011 famine.”

Of the four crises, Somalia’s is the most clearly linked to drought conditions, but insecurity caused by al-Shabab militants frequently keeps ­humanitarian workers from reaching civilians.

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Today’s coverage from Post correspondents around the world

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