President Donald Trump announced Monday he is withdrawing the US from the World Health Organization, a significant move on his first day back in the White House cutting ties with the United Nations’ public health agency and drawing criticism from public health experts.

Experts have also cautioned that withdrawing from the organization could weaken the world’s defenses against dangerous new outbreaks.

The U.S. has traditionally been the most generous benefactor of the WHO. A Trump executive order to cut ties with the WHO could pose a threat to global public health.