WASHINGTON - Six more U.S. senators on Friday backed bipartisan legislation to give President Joe Biden new powers to ban Chinese-owned video app TikTok on national security grounds as the company ramped up efforts to make its case ahead of a hearing next week.

This month, 12 senators led by Democrat Mark Warner and Republican John Thune unveiled legislation backed by the White House to give the Commerce Department new powers to regulate TikTok, which has more than 100 million U.S. users.

TikTok's critics fear its U.S. user data could be passed on to China's government by the app owned by the Chinese tech company ByteDance.

Senate Commerce Committee chair Maria Cantwell told Reuters she thinks most senators agree with the Warner-Thune bill and giving the administration new authority to review the security issues surrounding TikTok "is a good approach."

This week, TikTok said the Biden administration demanded that TikTok's Chinese owners divest their stake in the app or it could face a U.S. ban.

ByteDance confirmed this week 60% of its shares are owned by global investors and 20% by employees. It said only 20% are owned by its Chinese founders, although they have additional voting rights.

Biden's predecessor, Republican Donald Trump, had tried to ban TikTok in 2020 but U.S. courts blocked the attempt.

Nine Republicans and nine Democrats now sponsor the legislation, the Restricting the Emergence of Security Threats that Risk Information and Communications Technology (RESTRICT) Act. Separately, a source confirmed to Reuters the Justice Department has opened a criminal investigation after ByteDance said in December some employees improperly accessed U.S. TikTok user data of two journalists.

Reuters reported in December four ByteDance employees who were involved in the incident were fired, including two in China and two in the United States. Company officials said they were taking additional steps to protect user data.

ByteDance employees accessed the data as part of an unsuccessful effort to investigate leaks of company information this year, and were aiming to identify potential connections between two journalists.

"We have strongly condemned the actions of the individuals found to have been involved, and they are no longer employed at ByteDance," a ByteDance spokesperson said on Friday. "Our internal investigation is still ongoing, and we will cooperate with any official investigations."

Congress will get its first crack at questioning TikTok CEO Shou Zi Chew on March 23 when he testifies before the House Energy and Commerce Committee. He is in Washington preparing.

TikTok said content creators will come to Washington next week to make the case why the app should not be banned. "Lawmakers in Washington debating TikTok should hear firsthand from people whose lives would be directly affected by their decisions," TikTok said.

TikTok said it has spent more than $1.5 billion on rigorous data security efforts, rejects spying allegations and said "if protecting national security is the objective, divestment doesn't solve the problem: a change in ownership would not impose any new restrictions on data flows or access."

(Reporting by David Shepardson; editing by Diane Craft and David Gregorio)