In its ongoing campaign to extend its reach beyond its borders, the Chinese government has found a new form of leverage: American citizens in China.

Last year, Beijing prevented several U.S. citizens from leaving China, including a pregnant woman, according to email correspondence obtained by The Daily Beast. The total number of so-called exit bans placed on U.S. citizens in China is unknown, but at least two dozen cases have occurred within the past two years, according to one analyst’s estimate.

Chinese authorities typically target U.S. citizens of Chinese heritage for exit bans, usually in connection with an investigation. Sometimes, Beijing uses American citizens to try to coerce family members residing in the United States to return to China or to cooperate with Chinese authorities in investigations.

Chinese President Xi Jinping has championed a sweeping anti-corruption campaign with an international element, known as “Operation Fox Hunt,” aimed at pursuing Chinese citizens who have fled abroad after allegedly committing economic crimes. The United States does not have an extradition treaty with China and in the past has rarely cooperated with Chinese demands to repatriate Chinese citizens whom Beijing considers to be fugitives. Beijing has previously deployed undercover agents to the United States to coerce targets into returning to China, violating U.S. visa laws and prompting U.S. government indignation.

Now the People’s Republic seems to have found another lever of pressure. If one of Beijing’s targets living in the United States has relatives in China, Chinese authorities aren’t shy about applying pressure to those relatives, even if they are U.S. citizens. Exit bans are a “pretty new tool in the Chinese toolbox” for exerting such pressure, said John Kamm, founder of the U.S. nonprofit Dui Hua Foundation, which works on sensitive human rights cases in China.

“That individual might be treated as a material witness,” said Kamm. “Or that individual might be in effect being held as a hostage in an effort to get the people back.”

The Trump administration has pushed back quietly but firmly against exit bans. For example, in the lead-up to the first U.S.-China Law Enforcement and Cybersecurity Dialogue, held in Washington, D.C., in October 2017, Attorney General Jeff Sessions pushed for China to allow the free travel of three U.S. citizens who had been prevented from leaving China, including a pregnant woman, according to emails reviewed by The Daily Beast.

“Both sides will continue to cooperate to prevent each country from becoming a safe haven for fugitives and will identify viable fugitive cases for cooperation,” reads the U.S.-China joint statement released on Oct. 6, after the dialogue concluded. “Both sides commit to take actions involving fugitives only on the basis of respect for each other’s sovereignty and laws.”

It’s a delicate balancing act for an administration that also wishes to deport Chinese citizens who are in the United States illegally. In the past, China has often refused to accept deportations, leaving the United States with a large number of Chinese asylum seekers with final deportation orders. In 2015, Beijing’s refusal to accept deportees began to coincide with its push to repatriate fugitives it claimed were guilty of corruption. The Obama administration signed a memorandum of understanding with China to help expedite the deportation process, but remained reluctant to agree to Chinese demands to extradite fugitives.

Human rights groups have warned that fugitives may face torture or death back in China, also expressing concerns that Beijing might use trumped-up corruption charges to get their hands on troublesome political dissidents abroad.

The Department of Justice did not respond to emailed questions. The National Security Council did not respond to a request for comment.

The State Department declined to comment regarding the fate of those three U.S. citizens, citing privacy concerns, but a State Department spokesperson said that the U.S. government had not agreed to repatriate any Chinese citizen due to pressure from exit bans.

However, in January, the State Department warned Americans that going to China could be risky. “Exit bans have been imposed to compel U.S. citizens to resolve business disputes, force settlement of court orders, or facilitate government investigations,” states the travel advisory for U.S. citizens traveling to China, particularly U.S.-China dual nationals. “Individuals not involved in legal proceedings or suspected of wrongdoing have also be subjected to lengthy exit bans in order to compel their family members or colleagues to cooperate with Chinese courts or investigators.”

It’s difficult to know exactly how many U.S. citizens have been affected. The State Department declined to confirm the number of cases, citing privacy concerns, but Kamm said he knows of about two dozen cases over the past year and half alone.

“One of the problems with exit bans is that you don’t know that there is an exit ban on you until you actually get to the airport,” said Kamm. “There may be people in the country who have exit bans on them and they don’t know it.”

Exit bans have also been applied to ethnic Chinese of other nationalities. Swedish bookseller Gui Minhai was subject to an exit ban after he was kidnapped from Thailand and taken into custody in mainland China. The Swedish government has objected to his treatment there. Australian academic Feng Chongyi was interrogated by security officials while visiting China in 2017 then prevented from leaving the country. He was permitted to return to Australia a week later amid international media attention.

China has frequently used exit bans on its own citizens, most notably in 2010, when it prevented Chinese artist Ai Weiwei and others from traveling to Norway for Chinese activist Liu Xiaobo’s Nobel Peace Prize ceremony. Chinese authorities have also revoked the passports of many Uighurs, a Turkic-speaking ethnic minority in China’s northwest, as part of a massive repression campaign.

Exit bans violate United Nations human rights precepts. Article 13 of the Universal Declaration of Human Rights states that “everyone has the right to leave any country, including his own, and to return to his country.” The State Department declined to say whether or not China’s actions violated U.S. or international law; the Justice Department did not respond to request for comment.

“A lot of people simply don’t know that they can be stopped for leaving China,” said Kamm.

It’s well-known that China blacklists people from entering, denying visas to academics and journalists who are critical of Beijing.

But, Kamm said, “even worse is if you get into the country and they won’t let you out.”