Many of the immigrants had supported South Vietnam during the Vietnam War and the Vietnamese government would consider them a destabilizing force, Osius told Reuters.
"These people don't really have a country to come back to," he said.
Some of the immigrants had committed serious crimes, Osius acknowledged, although immigration advocates say that many of the convictions are decades old. Osius said that the repatriation agreement had meant that they would be left alone.
Immigration lawyers have said that some detained Vietnamese immigrants have been held for as long as 11 months because Immigration and Customs Enforcement cannot deport them.
Previously, arrested Vietnamese immigrants with final deportation orders who had arrived before 1995 would be released within 90 days, under supervision orders. In 2017, 71 Vietnamese people were deported to Vietnam, compared to 35 the previous year.
In February, several groups filed a class action lawsuit in Los Angeles federal court seeking to challenge the indefinite detentions.
One of those detained, Hoang Trinh, came to the U.S. in 1980 at the age of four when his family fled postwar Vietnam. He became a legal resident, married and raised two children in Orange County, Calif., the Washington Post reported in March.
He has spent at least seven months in detention under Immigration and Customs Enforcement. For a 2015 drug charge he spent a year in prison, then was arrested in 2017 for possession of marijuana. He was then ordered to be removed from the U.S. Trinh is a party to the lawsuit.
Phi Nguyen, litigation director with Asian Americans Advancing Justice--Atlanta, charged that Immigration and Customs Enforcement is acting "in complete disregard for the law."
"The only thing that has changed is that our administration wants the Vietnamese government to completely abandon the repatriation agreement."
Nguyen said that her parents fled Vietnam after her father was imprisoned for three years, during which he suffered from forced labor and starvation.
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The fate of these immigrants is a subject of international discussion. Katina Adams, a spokeswoman for the State Department's East Asia bureau, said the U.S. and Vietnamese governments continue to discuss their positions on Vietnamese citizens now in the U.S.
Reuters cited a senior Vietnamese official who said Vietnam needs to accept those who went to the U.S. after the war, not as a consequence of it.