UNITED NATIONS — A month ago, the secretary general of the United Nations, António Guterres, warned that 20 million people would fall into famine if his aid agencies could not corral $4.4 billion by the end of March.

It is almost the end of March, and so far, the United Nations has received less than a tenth of the money — $423 million, according to its Office for the Coordination of Humanitarian Affairs.

The funding appeal, and the paltry response, comes as the Trump administration is poised to make sharp cuts to its foreign aid budget, including for the United Nations. Historically, the United States has been the agency’s largest single donor for humanitarian aid.

For all four countries at risk — Nigeria, Somalia, South Sudan and Yemen — the United States has so far this year kicked in $277 million, not all of it for famine relief.

The conditions for famine are specific and not easy to meet, which is why the last time a famine was declared was in Somalia in July 2011, after 260,000 had died of hunger and related complications. The three criteria for declaring a famine are when one in five households in a certain area face extreme food shortages; more than 30 percent of the population is acutely malnourished; and at least two people for every 10,000 die each day.

A famine has already been declared in a swath of South Sudan. A similar risk looms over Somalia, still reeling from years of conflict, and Yemen, where Houthi insurgents are battling a Saudi-led coalition supported by the United States and Britain.

In northern Nigeria, a famine could already be underway, according to an early warning system funded by the United States Agency for International Development. But the security situation is so bad there that aid workers have been unable to assess levels of hunger.