Billionaire philanthropist Bill Gates attends the World Economic Forum annual meeting in Davos, Switzerland Jan. 19, 2017. REUTERS/Ruben Sprich

Bill Gates, the richest man on Earth, is worried about President Donald Trump's promise to put America first at all costs. In an interview with USA Today published Tuesday, the philanthropist and longtime advocate for global health initiatives said his foundation, the Bill and Melinda Gates Foundation, will attempt to push the new government away from such stark nationalism.

Gates especially brought up concerns in Africa, where much of his foundation's work has focused. "If you interpret America First in certain ways, it would suggest not prioritizing the stability of Africa and American leadership," Gates told USA Today.

The billionaire met with Trump before he took office. "He admires people who are successful, so he couldn't have been nicer and he listened," Gates said, before adding that other folks surrounding the president could shape his policy. Amid the fallout of the United Kingdom's vote last year to leave the European Union and Trump's stated nationalism, Gates said he was worried that powerhouses that typically contribute large sums to global health initiatives will soon withdraw.

"The budget is particularly tight, people are talking about increasing defense [spending], lowering taxes, interest costs will be higher," he said to USA Today. "So when you look at it mathematically you say, 'Will the saving of millions of lives for less than $100 a year of drugs, will the U.S. continue to do that?' It's not clear where we're headed."

Gates also expressed to the Guardian deep concerns about the Trump administration's decision to block all U.S. funds to any organization that is involved with providing abortion services. It could have serious effects on women and girls' health worldwide, he said. Trump's order went further than those of past Republican presidents and could remove critical funding for broad health initiatives.

"The U.S. is the No. 1 donor in the work that we do," Gates told the Guardian. "Government aid can't be replaced by philanthropy. When government leaves an area like that, it can't be offset, there isn't a real alternative. This expansion of this policy, depending on how it's implemented, could create a void that even a foundation like ours can't fill."