TikTok CEO Kevin Mayer resigned Thursday amid U.S. pressure for its Chinese owner to sell the popular video app, which the White House says is a security risk.

In a letter to employees, Mayer said that his decision to leave comes after the “political environment has sharply changed.”

His resignation comes after U.S. President Donald Trump ordered a ban on TikTok, unless parent company Bytedance sells its U.S. operations to an American company within 90 days.

MORE: Microsoft in advanced talks to buy TikTok's US business 

“I have done significant reflection on what the corporate structural changes will require, and what it means for the global role I signed up for,” he said in the letter. “Against this backdrop, and as we expect to reach a resolution very soon, it is with a heavy heart that I wanted to let you all know that I have decided to leave the company.”

Mayer, a former Disney executive, joined TikTok as CEO in May.

“We appreciate that the political dynamics of the last few months have significantly changed what the scope of Kevin’s role would be going forward, and fully respect his decision. We thank him for his time at the company and wish him well,” TikTok said in a statement.

Earlier this month, Trump ordered a sweeping but unspecified ban on dealings with the Chinese owners of consumer apps TikTok and WeChat as the U.S. heightens scrutiny of Chinese technology companies, citing concerns that they may pose a threat to national security.

Bytedance is currently in talks with Microsoft for the U.S. firm to buy TikTok's U.S. operations.

By Zen Soo, AP technology writer