The US Agency for International Development (USAID) has officially closed its doors after President Donald Trump gradually dismantled the agency over its allegedly wasteful spending.

More than 80% of all the agency's programmes were cancelled as of March, and on Tuesday, the remainder were formally absorbed by the state department.

The shuttering of USAID, which administered aid for the US government, the world's largest such provider, has been newly criticised by former Presidents Barack Obama and George W Bush.

These aid cuts could cause more than 14 million additional deaths by 2030, according to a warning published by researchers in the Lancet medical journal.

The authors of the Lancet report called the numbers "staggering", and projected that a third of those at risk of premature deaths were children.

A state department official said the study used "incorrect assumptions" and insisted that the US would continue to administer aid in a "more efficient" way, the AFP news agency reported.

Founded in 1961, USAID previously employed some 10,000 people, two-thirds of whom worked overseas, according to the Congressional Research Service.

The controversial cuts began early in Trump's second term, when billionaire and former presidential adviser Elon Musk was tasked with shrinking the federal workforce.

The move was widely condemned by humanitarian organisations around the world.

Among the programmes that were curbed were efforts to provide prosthetic limbs to soldiers injured in Ukraine, to clear landmines in various countries, and to contain the spread of Ebola in Africa.

On Wednesday morning, the agency's website continued to display a message saying that all USAID direct-hire personnel globally had been placed on administrative leave from 23 February.

Secretary of State Marco Rubio previously said that the remaining 1,000 programmes after the cuts would be administered under his department.

"This era of government-sanctioned inefficiency has officially come to an end," he added on Tuesday.

"Under the Trump Administration, we will finally have a foreign funding mission in America that prioritises our national interests," he wrote in a post on Substack.

Trump has repeatedly said he wants overseas spending to be closely aligned with his "America First" approach.

Bush and Obama delivered their messages of condemnation in a video conference they hosted with U2 singer Bono for thousands of members of the USAID community.

Bush, a fellow member of Trump's Republican Party, focused on the impact of cuts to an AIDS and HIV programme that was started by his administration and subsequently credited with saving 25 million lives.

"You've shown the great strength of America through your work - and that is your good heart,'' Bush told USAID workers in a recorded statement, according to US media. "Is it in our national interests that 25 million people who would have died now live? I think it is, and so do you."

Meanwhile, Obama, a member of the opposition Democratic Party, affirmed the work that USAID employees had already done.

"Gutting USAID is a travesty, and it's a tragedy. Because it's some of the most important work happening anywhere in the world," Obama was quoted as saying.

Long-time humanitarian advocate Bono spoke about the millions of people who he said could die because of the cuts.

"They called you crooks when you were the best of us," he told attendees of the video conference.

USAID was seen as integral to the global aid system. After Trump's cuts were announced, other countries followed suit with their own reductions - including the UK, France and Germany.

Last month, the United Nations said it was dealing with "the deepest funding cuts ever to hit the international humanitarian sector".