Amazon has instructed its employees to remove the short-video app TikTok from their devices immediately, according to a person familiar with the matter. In an email sent to employees Friday morning and obtained by CNN, Amazon (AMZN) said that due to “security risks, the TikTok app is no longer permitted on mobile devices that access Amazon email.

“Accessing TikTok from company laptops is still permitted, the email said, but mobile devices will be cut off from Amazon email unless the app is removed. Amazon didn’t immediately respond to a request for comment.

In a statement, a TikTok spokesperson said Amazon “did not communicate to us before sending their email, and we still do not understand their concerns.” “We welcome a dialogue so we can address any issues they may have and enable their team to continue participating in our community,” the spokesperson added. “We’re proud that tens of millions of Americans turn to TikTok for entertainment, inspiration, and connection, including many of the Amazon employees and contractors who have been on the frontlines of this pandemic.”

The announcement marks the biggest company yet to warn its employees of TikTok as a possible security risk, following claims by US politicians that the app could undermine national security given its ties to China. Cybersecurity experts have been more cautious about that assessment.