Washington DC - A federal judge on Monday declared that President Donald Trump's cuts to foreign aid contracts and grants could go ahead, but said that payments must be made on completed projects.

While the judge will not order Trump to restore canceled contracts and cannot compel payments, ultimately, close to $2 billion must be paid for work that had already been completed prior to the cuts.

US District Judge Amir Ali, in a separate ruling, said that the Trump administration cannot overrule spending commitments that have been approved by Congress.

"The provision and administration of foreign aid has been a joint enterprise between our two political branches," Ali wrote in the ruling.

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"That partnership is built not out of convenience, but of constitutional necessity," he said. "Today, this Court reaffirms these firmly established principles of our Constitution."

The rulings are in response to a number of lawsuits filed since Trump systematically tore apart the US Agency for International Development (USAID), firing tens of thousands of staff and cutting thousands of contracts.

Many of the contracts that have been cut were going to projects that feed and look after malnourished children. Others were directed at natural and humanitarian disaster assistance, refugee camps for displaced people, and more support for the world's most vulnerable.

One project in Nigeria provided treatment to 5.6 million starving children and 1.7 million women. The cut puts about 60,000 children under the age of 5 at risk of immediate starvation and death.

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"Today's decision affirms a basic principle of our Constitution: the president is not a king," said Lauren Bateman, an attorney with Public Citizen Litigation Group and a lawyer representing plaintiffs in the case against Washington.

"We are painfully aware that, without unwinding the mass termination of foreign assistance awards, winning on the constitutional issues does not avert the humanitarian disaster caused by the Trump Administration's freeze on foreign assistance," she said in a statement.

"And it does not undo the damage that the freeze has already inflicted on millions of vulnerable people across the world. Deaths will continue to mount."