The State Department renewed a travel advisory Thursday urging Americans to "exercise increased caution" in China. "Chinese authorities have asserted broad authority to prohibit U.S. citizens from leaving China by using 'exit bans,' sometimes keeping U.S. citizens in China for years," the advisory said.

A previous advisory for the country issued in January 2018 similarly encouraged Americans to "exercise increased caution" due to "the arbitrary enforcement of local laws" and "special restrictions on dual U.S.-Chinese nationals," according to Reuters.

But the new advisory adds, "Extra security measures, such as security checks and increased levels of police presence, are common in the Xinjiang Uighur and Tibet Autonomous Regions. Authorities may impose curfews and travel restrictions on short notice."

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The advisory said exit bans are used to compel Americans to take part in Chinese government probes, coax people back to China from overseas and "aid Chinese authorities in resolving civil disputes in favor of Chinese parties."

The advisory comes weeks after China detained two Canadian nationals. Former Canadian diplomat Michael Kovrig and entrepreneur Michael Spavor were taken into custody in December on suspicion of "engaging in activities that endanger" China's national security.

Their detentions came after police in Canada arrested Huawei chief financial officer Meng Wanzhou. She was arrested Dec. 1 at the request of the U.S., which is seeking charges against Meng for allegedly violating U.S. sanctions against Iran.