The sudden freezing of U.S. aid to malaria projects comes as deadly new variants are spreading in Africa and could have a devastating impact, the head of a major NGO has said.

The U.S. government has provided some 40% of the annual funding globally for control and research into a disease that causes more than 600,000 deaths from 250 million cases each year — mostly in Africa.

That funding, of up to $1 billion a year, is now frozen as part of U.S President Donald Trump's plan to axe foreign aid.