U.S.-bound flights carrying evacuees from Afghanistan are on hold after four cases of measles were diagnosed among Afghans who had already arrived in the country, the White House said Friday.

The flights have been temporarily paused “out of an abundance of caution” at the request of the Centers for Disease Control and Prevention, White House press secretary Jen Psaki told reporters.

The Afghans diagnosed with the measles are in quarantine in accordance with public health guidelines, and the CDC has begun contact-tracing steps, Psaki said.

The CDC did not immediately respond to CNBC’s request for comment on Psaki’s remarks.

Psaki noted that all those arriving as part of “Operation Allies Welcome,” the Biden administration’s ongoing effort to resettle Afghan allies following the U.S. full withdrawal from Afghanistan, are required to be vaccinated for the measles as a condition for entry.

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