The State Department issued an updated travel advisory for China Thursday, warning Americans to "Exercise increased caution ... due to arbitrary enforcement of local laws as well as special restrictions on dual U.S.-Chinese nationals."

The guidance warned that authorities in Beijing have used so-called "exit bans" to keep U.S. citizens from leaving China, sometimes holding them "for years."

"U.S. citizens may be detained without access to U.S. consular services or information about their alleged crime," the department said. "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 U.S. citizens for sending private electronic messages critical of the Chinese government."

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The State Department added that exit bans are used to force Americans to cooperate in Chinese government investigations, to "lure individuals back to China from abroad," and to help authorities resolve civil disputes "in favor of Chinese parties."

"In most cases, U.S. citizens only become aware of the exit ban when they attempt to depart China, and there is no method to find out how long the ban may continue," said the advisory, which also warned that Americans under exit bans had been "harassed and threatened."

The updated guidance was issued amid increasing tensions between the U.S. and China over ongoing trade disputes, as well as the Dec. 1 arrest of Huawei Technologies Chief Financial Officer Meng Wanzhou in Vancouver.

The State Department urges travelers to China to keep a U.S. passport with a valid Chinese visa close at hand. If arrested, U.S. citizens should ask police or prison officials to notify the U.S. Embassy in Beijing or the nearest consulate immediately.