"Chinese authorities have asserted broad authority to prohibit USA citizens from leaving China by using 'exit bans, ' sometimes keeping US citizens in China for years", the State Department advisory read.
Updating its travel advisory, the State Department maintained its previous guidance that Americans should "exercise increased caution" in China and stopped short of discouraging visits. The ban specifically targets U.S.
"U.S. citizens may be subjected to prolonged interrogations and extended detention for reasons related to 'state security.' Security personnel may detain and/or deport US citizens for sending private electronic messages critical of the Chinese government", the advisory said. "Chinese citizens and USA citizens of Chinese heritage may be subject to additional scrutiny and harassment".
The advisory notes that China has increasingly started to coercively use "exit bans" in order to keep USA citizens in the country, sometimes for years, for the purposes of compelling them to participate in Chinese government investigations or of luring other individuals back to China from overseas. However, they have since been prevented from leaving the country under exit bans.
Separately, three U.S. citizens were accused of committing "economic crimes" and barred from leaving China in November.
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US and Chinese flags are seen as Secretary of State Mike Pompeo and China's Foreign Minister Wang Yi meet at the US Department of State 23 May 2018 in Washington, DC.
China and Canada both said the case was different to that of Mr Kovrig and Mr Spavor who stand accused of harming national security.
The updated guidance was issued amid increasing tensions between the US and China over ongoing trade disputes, as well as the December 1 arrest of Huawei Technologies Chief Financial Officer Meng Wanzhou in Vancouver.
U.S. prosecutors have accused her of misleading banks about transactions linked to Iran, putting the lenders at risk of violating Washington's sanctions.
On Thursday, China's prosecutor general said the pair had "violated our country's laws and regulations" and were being investigated.