WASHINGTON, D.C.—The U.S. State Department on Wednesday will stop issuing certain kinds of visas to some citizens of Cambodia, Eritrea, Guinea and Sierra Leone because the nations are not taking back their citizens the United States wants to deport.

The new policies, laid out in State Department cables reviewed by Reuters on Tuesday and described in a department news briefing, are the latest example of U.S. President Donald Trump’s effort to crack down on immigrants who are in the United States illegally.

The cables, sent by Secretary of State Rex Tillerson to consular officials around the world, said the four countries were “denying or unreasonably delaying” the return of their citizens, and that visa restrictions would be lifted in a country if it accepted its deportees.

“The Secretary determines the categories of applicants subject to the visa restrictions, and the categories differ slightly country by country,” State Department spokeswoman Heather Nauert said in the news briefing on Tuesday.

The visa sanctions vary in severity, with Eritrea facing the harshest ones. Any Eritreans who apply in their own country for most U.S. business or tourist visas will be rejected, according to one of the cables.

In Guinea, the United States will no longer issue a range of tourist, business and student visas to government officials and their immediate family members who apply from inside the country, another cable said.

In Cambodia, the sanction is tailored. Only Foreign Ministry employees at or above the rank of director general, and their families, who apply inside the country will be barred from getting some visas for personal travel, a third cable said.

For Sierra Leone, only Foreign Ministry and immigration officials will be denied tourist and business visas at the U.S. Embassy in Freetown, according to a fourth cable.

“American citizens have been harmed because foreign governments refuse to take back their citizens,” Thomas Homan, acting director of U.S. Immigration and Customs Enforcement, said in a Department of Homeland Security statement.

The new rules, which go into effect on Wednesday, do not affect visas that already have been granted.