The U.S. State Department on Thursday updated its travel warning about China, urging Americans to "exercise increased caution" in the country "due to arbitrary enforcement of local laws as well as special restrictions on dual U.S.-Chinese nationals."

The State Department's "Level 2" warning, which was first issued last year, noted that Chinese authorities have "exit bans" to prevent U.S. citizens from leaving China, sometimes "for years."

The updated warning notes that the bans are used "coercively" to "lure" people back to China from abroad, a point that was not mentioned when the warning was first issued. The bans are also used compel Americans to participate in Chinese government investigations and to help Chinese authorities resolve civil disputes "in favor of Chinese parties," the warning says.

"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. U.S. citizens under exit bans have been harassed and threatened," the warning said.