Donald Trump has ordered a communications blackout at America's federal health agencies, according to reports.

The CDC, FDA, HHS and NIH have all been told to pause external communications, including publishing scientific reports, updating websites or issuing health advisories.

The directive came without warning, sources told the Washington Post, and with little guidance as to how long it may last.

The health agencies play a vital role in gathering and sharing critical information with the public, including on outbreaks of infectious diseases, raising the alarm over foodborne disease outbreaks and food recalls.