Three unaccompanied migrant children in U.S. government custody have tested positive for the coronavirus, federal officials said Thursday, highlighting concerns among advocates about the vulnerability of detained immigrants during the global pandemic.

The three minors, who are housed in a shelter in New York, are the first confirmed coronavirus cases among the 3,600 unaccompanied children in the custody of the Office of Refugee Resettlement, or ORR. In response to the outbreak, the refugee agency has stopped releasing migrant children in New York facilities to sponsors, who are typically family members living in the U.S.

No further discharge restrictions have been made as of Thursday, an ORR spokesperson said. The agency had previously stopped placing minors in shelters and programs in California, New York and Washington. In addition to the three confirmed cases, ORR has tested 15 other children in its custody as of Thursday. Eleven results came back negative and the rest are pending. The agency is consulting with local health authorities in New York to "determine next steps."

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Officials also revealed on Thursday that the number of positive coronavirus cases among staff members and contractors at facilities for unaccompanied migrant children has grown to seven. Six of them work in New York and one is a staff member in Texas. A foster parent in Washington also tested positive for the virus earlier this month. The refugee agency said it is also notifying anyone who may have been exposed to the workers.

Coronavirus is particularly dangerous for older people and those with underlying medical issues, but children and young people can carry and transmit the virus, even if the risk of serious illness is relatively low. Migrant minors in ORR custody crossed the southern border without parents or guardians, or in certain circumstances, were separated from them.

In this September 24, 2019, photo, girls play dominos with a staff member at a shelter for migrant teenage girls, in Lake Worth, Florida. Wilfredo Lee / AP

The announcement on Thursday is likely to fuel even more calls for the Trump administration to quickly release some of the tens of thousands of immigrants it is currently detaining, especially as the public health crisis to contain the coronavirus intensifies. On Wednesday, lawyers asked a federal court in California to require officials to release unaccompanied migrant children who have been in government custody for more than a month or transfer them to facilities where social distancing can be reasonably practiced.

Similar calls have been made for Immigration and Customs Enforcement, or ICE, to reduce its detainee population, which stood at more than 38,000 over the weekend. Despite confirmed coronavirus cases among detainees and staff at detention facilities, the agency has not moved to significantly downsize its detainee population.

In response to the pandemic, ORR, an agency within the Department of Health and Human Services, is also prioritizing placing migrant children in local facilities after receiving them from Department of Homeland Security officials, who are generally the first to apprehend unaccompanied minors near the southern border. Officials said the move is designed to avoid air travel.

Last week, ORR leadership also issued guidance requiring that migrant children in its custody undergo mandatory temperature checks twice day, agency officials said. If a child has a temperature higher than 100 degrees, the shelters have to notify the government.

Under U.S. law, the Department of Homeland Security is generally required to transfer unaccompanied migrant children in its custody to ORR within 72 hours. The children stay in ORR custody until they are released to sponsors.

Two years ago, the agency struggled to house the thousands of minors who the Trump administration separated from their parents or guardians as part of the controversial "zero tolerance" policy.