By Katie Paul and Lisa Richwine

(Reuters) - Walt Disney Co's <DIS.N> top streaming executive, Kevin Mayer, will leave the entertainment and theme parks giant to become the chief executive officer of TikTok, the popular video app owned by China's ByteDance Technology Co, the companies said on Monday.

Mayer led the successful launch of the Disney+ streaming service in November but in February was passed over as Disney's new chief executive.

Mayer's appointment will be effective June 1, when he will also become chief operating officer of ByteDance, the Chinese company said.

TikTok, which allows users to create short videos with special effects, has become wildly popular with U.S. teenagers doing viral challenges that pair dances with music clips from the app's library. TikTok has hinted at ambitions to build a music streaming business, announcing in January that it was partnering with U.K.-based music rights agency Merlin to expand its musical selections.

ByteDance's Chinese ownership, however, has sparked concerns in Washington about TikTok's handling of personal data. The company uses sophisticated artificial intelligence to make video recommendations based on users' behavior on the app.

In November, the U.S. government launched a national security review of ByteDance's $1 billion acquisition of social media app Musical.ly, which became TikTok. Two senators introduced a bill to ban federal employees from using TikTok on government-issued phones.

One of those senators, Republican Josh Hawley, said TikTok previously told him its executives could not testify before Congress because they were located in China.

"But this new executive lives in the USA," Hawley wrote on Twitter on Monday. "I look forward to hearing from him. Under oath."

To appease concerns, ByteDance has stepped up efforts to separate TikTok from much of its Chinese businesses and has made several high-profile executive hires in recent months. It appointed former Microsoft intellectual property chief Erich Andersen as global general counsel in January, after hiring Vanessa Pappas, a veteran YouTube executive, to run its U.S. operations last year.

Speculation over Mayer's future began swirling in February after Disney named Robert Chapek as chief executive officer. Mayer, who has a "loud and forceful" style, according to a former Disney executive, was seen as a dealmaker who had only recently been put in charge of a large profit-and-loss division. His relative lack of operating experience was a main reason he did not get the top job, the former executive said.