“There’s more pressure on programmes like the World Food Programme and others than there has been in decades,” he said.

Richard Gowan, a UN expert at the European Council on Foreign Relations, said high-profile emergencies would still attract funding at the expense of less well-known trouble spots.

“The Europeans will always find enough to keep the Syrian relief operations going even if there are shortfalls,” he said. “What I rather imagine is that funds for dealing with foreign crises about which we know little, like the crisis in Central African Republic or parts of the Sahel, which are suffering very badly from violence and potential famine, there I think the money would start to dry up.”

At the same time, Mr Trump has managed to end speculation about another financial issue.

His spokesman, Sean Spicer, announced that the president would be donating his salary to a cause selected by White House correspondents.

“I think his view is he made a pledge to the American people he wants to donate it to charity and he'd love your help to determine where it should go,” he said during his regular briefing.

Mr Trump, who is believed to be worth $3billion, promised during the campaign that he would give his $400,000 White House pay cheque to charity without offering any explanation about how it would work – until now.