Health officials working at agencies housed within the Department of Health and Human Services have been told to suspend their work with the World Health Organization immediately after President Donald Trump signed an executive order pulling the country out of the organization.

Centers for Disease Control and Prevention official John Nkengasong sent a memo to senior agency leaders Sunday night in which he said that all agency staff who work with the WHO must stop their collaborations effective immediately and “await further guidance.” 

“All CDC staff engaging with WHO through technical working groups, coordinating centers, advisory boards, cooperative agreements or other means — in person or virtual,” must end their work with the WHO according to the memo. Nkengasong’s memo additionally said CDC staff are not allowed to visit WHO offices.

The memo comes after Trump, in addition to dozens of other executive orders, pulled the U.S out of the WHO last week. The Trump administration additionally ordered health agencies to suspend all public communications.

Trump has long been critical of the WHO_, saying it gets too much money from the United States, which is the top donor to the organization compared to every other country.

Some health experts said the sudden stoppage of collaboration with the WHO was a surprise and would set back work on investigating and preventing outbreaks of various diseases domestically and abroad. 

“Stopping communications and meetings with WHO is a big problem,” Dr. Jeffrey Klausner, a University of Southern California public health expert who collaborates with the WHO, told NBC News. “People thought there would be a slow withdrawal. This has really caught everyone with their pants down.”

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The memo also comes as health authorities worldwide are closely watching bird flu outbreaks among U.S. livestock. 

California reported the first case of a subvariant of bird flu in a commercial operation, and Georgia shut down all of its commercial poultry plants after it was detected in one commercial operation. Earlier this month, Louisiana reported the first known human death in the U.S. from bird flu.