NEW YORK/SAN FRANCISCO (Reuters) - The U.S. Attorney General on Tuesday struck down a decision that had allowed some asylum seekers to ask for bond in front of an immigration judge, in a ruling that expands indefinite detention for some migrants who must wait months or years for their cases to be heard.

The first immigration court ruling from President Donald Trump’s newly appointed Attorney General William Barr is in keeping with the administration’s moves to clamp down on the asylum process as tens of thousands of mostly Central Americans cross into the United States asking for refuge. U.S. immigration courts are overseen by the Justice Department and the Attorney General can rule in cases to set legal precedent.

Barr’s ruling is the latest instance of the Trump administration taking a hard line on immigration. This year the administration implemented a policy to return some asylum seekers to Mexico while their cases work their way through backlogged courts, a policy which has been challenged with a lawsuit.

Several top officials at the Department of Homeland Security were forced out this month over Trump’s frustrations with an influx of migrants seeking refuge at the U.S. southern border.

Barr’s decision applies to migrants who crossed illegally into the United States.

Typically, those migrants are placed in “expedited removal” proceedings - a faster form of deportation reserved for people who illegally entered the country within the last two weeks and are detained within 100 miles (160 km) of a land border. Migrants who present themselves at ports of entry and ask for asylum are not eligible for bond.

But before Barr’s ruling, those who had crossed the border between official entry points and asked for asylum were eligible for bond, once they had proven to asylum officers they had a credible fear of persecution.

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“I conclude that such aliens remain ineligible for bond, whether they are arriving at the border or are apprehended in the United States,” Barr wrote.

Barr said such people can be held in immigration detention until their cases conclude, or if the Department of Homeland Security (DHS) decides to release them by granting them “parole.” DHS has the discretion to parole people who are not eligible for bond and frequently does so due to insufficient detention space or other humanitarian reasons.

Barr said he was delaying the effective date by 90 days “so that DHS may conduct the necessary operational planning for additional detention and parole decisions.”

The decision’s full impact is not yet clear, because it will in large part depend on DHS’ ability to expand detention, said Steve Vladeck, a law professor at the University of Texas.

“The number of asylum seekers who will remain in potentially indefinite detention pending disposition of their cases will be almost entirely a question of DHS’s detention capacity, and not whether the individual circumstances of individual cases warrant release or detention,” Vladeck said.

DHS officials did not immediately respond to a request for comment on the decision. The agency had written in a brief in the case arguing that eliminating bond hearings for the asylum seekers would have “an immediate and significant impact on...detention operations.”

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In early March, Immigration and Customs Enforcement (ICE), the DHS agency responsible for detaining and deporting immigrants in the country illegally, said the average daily population of immigrants in detention topped 46,000 for the 2019 fiscal year, the highest level since the agency was created in 2003. Last year, Reuters reported that ICE had modified a tool officers have been using since 2013 when deciding whether an immigrant should be detained or released on bond, making the process more restrictive.

The decision will have no impact on unaccompanied migrant children, who are exempt from expedited removal. Most families are also paroled because of a lack of facilities to hold parents and children together.

Michael Tan, from the American Civil Liberties Union, said the rights group intended to sue the Trump administration over the decision, and immigrant advocates decried the decision.

Barr’s decision came after former Attorney General Jeff Sessions decided to review the case in October. Sessions resigned from his position in November, leaving the case to Barr to decide.