Vietnamese citizens who are waiting for marriage documents in America with an expired visa need to be careful, California-based Tania Pham warns as a nationwide wave of arrests of immigrants facing deportation began on Sunday.

She told VnExpress: "The U.S. Immigration and Customs Enforcement (ICE) knows the addresses of these individuals."

The sweeps are expected to target hundreds of families in 10 cities that have recently been ordered deported by an immigration court but have not yet left the country, Reuters reported last Friday.

The operation was announced by President Donald Trump on Twitter last month, but postponed because Democrats and Republicans decided to work together to devise a solution to the immigration crisis.

Trump decided to resume the crackdown when the U.S. Congress failed to strike a deal with his administration.

The major cities on ICE's list of raids in June, Atlanta, Baltimore, Chicago, Denver, Houston, Los Angeles, Miami, New York and San Francisco have confirmed that they did not cooperate with the ICE in arresting illegal immigrants because it violated their rights.

"Some city-level officials have instructed immigrants not to open the door when ICE employees visit. Vietnamese people can do that because the law allows them to," said Texas-based attorney David Nguyen.

Nguyen said that ICE’s targets are undocumented immigrants who have been ordered to leave by the U.S. courts but still remain in the country, Nguyen said, adding that they are in the process of being deported from the U.S. and are allowed to leave voluntarily.

"When they have the right to leave voluntarily, the immigrants will be directed to the embassy or consulate of their origin country where they get a confirmation paper saying they will leave the U.S.," Nguyen said.

He said the U.S. government did not publish a list of these immigrants targeted by the crackdown, so it is unclear how many Vietnamese people are at risk. He said ICE will go after undocumented individuals starting at the latest address that it has knowledge of.

All immigrant rights groups in the U.S. have advised people to "not open the door" to protect themselves, lawyer Pham said. United We Dream, an NGO, has called on immigrants to exercise their right to remain silent, to not sign any documents and call their hotline number for assistance.

Pham said some Vietnamese have been arrested by the authorities because their visas had expired. She said around 10,000 Vietnamese will be affected as the Trump administration tightens its control on immigrants.

The U.S. Census Bureau estimates that about 1.3 million Vietnamese immigrants are living in the United States. Many are at risk of deportation, especially those who have lost their green card due to convictions.