The U.S. State Department’s Jan. 3 warning follows China’s actions in recent months to prohibit visitors with U.S. passports from leaving the country for a variety of reasons. This includes effectively banning members of a family from China to lure a relative back to the country.

China’s relations with the U.S. and with Canada deteriorated following the arrest in Vancouver, British Columbia, of a top executive with Huawei, (HUAWEI) a leading Chinese electronics and telecom firm, on a U.S. warrant. American officials claim that Meng Wanzhou violated U.S. sanctions on trade with Iran by misleading American financial institutions into facilitating the deals. China has no prohibition on commerce with Iran.

In November, The New York Times reported that Liu Changming’s two children were barred from leaving, and his wife allegedly held in a secret prison. All three are American citizens, as China doesn’t recognize dual citizenship. The government has treated them as Chinese citizens, however. Liu has been accused of massive fraud by Chinese officials.

The state department also said that China might force a U.S. citizen or U.S.-Chinese dual citizen to remain in order to gain cooperation with a government investigation, or to tamper with the outcome of a civil dispute towards Chinese parties.

China has detained 13 Canadian citizens since the arrest, according to The Globe and Mail with eight or more subsequently released. A spokesperson for the Canadian foreign ministry told The New York Times that it didn’t believe the arrests were retaliatory. Canada has not issued a travel advisory. China is Canada’s third-biggest trading partner, though dwarfed by the U.S., which trades eight times that amount, and comprises 50% of all Canadian imports and exports.