The U.S. State Department issued a new travel warning for China on Thursday.
A level 2 advisory urging visitors from the U.S. to exercise increased caution has been in place since last year.
But the updated advisory warns China can keep U.S. citizens in the country using so-called "exit bans." The department says the bans can be used to lure U.S. citizens back into China, or to compel them to stay in the country because of Chinese government investigations or civil disputes.
The advisory says exit bans let the Chinese government hold U.S. citizens for long periods of time, sometimes years, and can also prevent travelers from accessing U.S. consular services.
The department also warns China doesn't recognize dual nationality, meaning U.S.-Chinese citizens and U.S. citizens of Chinese heritage could face additional scrutiny and harassment.
Tensions with China have been high since the country detained two Canadian citizens for allegedly "engaging in activities that endanger [China's] national security." The Canadian government has said those arrests were retaliation for its detainment of Huawei CFO Meng Wanzhou, who faces extradition to the U.S.
The State Department has issued a number of level 2 travel advisories in recent weeks. In December alone, 17 were issued for countries including Burma, France, Italy and Spain.