The government argued in its filing that TikTok under Chinese ownership poses a serious national security threat because of its access to vast personal data of Americans, asserting China can covertly manipulate information that Americans consume via TikTok.
"The serious national-security threat posed by TikTok is real," the department said on Friday.
"TikTok provides the Chinese government the means to undermine US national security in two principal ways: data collection and covert content manipulation."
The Biden administration asked the US Court of Appeals for the District of Columbia to reject lawsuits by TikTok, parent company ByteDance and a group of TikTok creators seeking to block the law that could ban the app used by 170 million Americans.
TikTok has repeatedly denied it would ever share US user data with China or that it manipulates video results.
"The government has never put forth proof of its claims, including when Congress passed this unconstitutional law. Today, once again, the government is taking this unprecedented step while hiding behind secret information", TikTok posted on X.
The department's filing details wide-ranging national security concerns about ByteDance's ownership of TikTok.
"China's long-term geopolitical strategy involves developing and pre-positioning assets that it can deploy at opportune moments," the department said.
The government acknowledged in a separate declaration it had no information that the Chinese government had gained access to the data of US TikTok users but said the risk was too great.
The government also filed a classified document detailing additional security concerns about ByteDance's ownership of TikTok, as well as declarations from the FBI, Office of the Director of National Intelligence and DOJ's National Security Division.
Signed by President Joe Biden in April, the law gives ByteDance until January 19 to sell TikTok or face a ban.
The White House says it wants to see Chinese-based ownership ended on national security grounds, but not a ban on TikTok.
The department rejected all the arguments raised by TikTok, including that the law violates the free-speech rights of Americans who use the short video app, saying the law addresses national security concerns, not speech, and is aimed at China's ability to exploit TikTok to access Americans' sensitive personal information.
The DOJ said TikTok's $US2 billion ($A3 billion) plan to protect US user data was insufficient in part because US officials were not confident ByteDance would be able to catch violations.
The appeals court will hold oral arguments on September 16, putting the issue of TikTok's fate into the final weeks of the November 5 presidential election.
Republican presidential nominee Donald Trump has joined TikTok and said in June he would never support a TikTok ban.
Vice-President Kamala Harris, who is poised to become the Democratic nominee, joined TikTok this week.
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