“Exercise increased caution in the People’s Republic of China (PRC) due to arbitrary enforcement of local laws for purposes other than maintaining law and order,” State said in a travel advisory on its website that was also emailed to U.S. citizens registered as being in China. “This arbitrary enforcement may include detention and the use of exit bans.”

The warning, sent Saturday morning U.S. time, says that Chinese security forces may “detain and/or deport” American citizens for “sending private electronic messages critical of” China’s government.

“U.S. citizens may be detained without access to U.S. consular services or information about their alleged crime. U.S. citizens may be subjected to prolonged interrogations and extended detention for reasons related to ‘state security,’” the warning added.

Tensions between the U.S. and China have risen in the wake of the worldwide coronavirus pandemic. President Donald Trump and other administration officials have blamed China for taking insufficient actions to contain the spread of Covid-19.

Trump said Friday that the U.S.-China relationship is “severely damaged,” and that he is no longer thinking about a phase two trade deal to ease a tit-for-tat tariff standoff between the two countries.

The U.S. has condemned China’s moves to crack down in dissent in Hong Kong by enacting a sweeping security law and is moving to ban federal contractors from using products from Chinese telecom giant Huawei.

China, in turn, has vowed “firm countermeasures” after the U.S. this week slapped sanctions on sitting Communist Party officials for the first time over human rights abuses in the western region of Xinjiang. The U.S. move was seen as a major escalation in the Trump administration’s increasingly tense rivalry with Beijing.