Vehicles wait in line to cross into Canada at the Peace Bridge Plaza on March 18, 2020, in Buffalo N.Y. The Canada-U.S. border will be closed to non-essential traffic in both directions "by mutual consent," President Donald Trump confirmed Wednesday, as efforts across the continent to contain the widening COVID-19 pandemic continued to upend daily life in North America.

Updated at 9:57 p.m. ET

President Trump plans to seal off the U.S-Mexico border to migrants under a law intended to protect the country from communicable disease — a move that comes as the U.S. immigration system grinds to a halt in response to the growing coronavirus pandemic.

At a press conference Wednesday, Trump said the southern border would not close completely. But the move would allow the administration to quickly deport asylum-seekers and other migrants who cross the U.S.-Mexico border illegally without due process.

Meanwhile, the U.S. and Canada have agreed to close their mutual border to all "non-essential" traffic, following earlier restrictions imposed by the Trump administration on travel from Europe and China.

The dramatic developments on the northern and southern borders come as immigration is being curtailed in other ways, too.

U.S. Immigration and Citizenship Services, the agency in charge of processing green card and citizenship applications as well as conducting asylum interviews, is closing its field offices to the public. Immigrants fighting deportation are having their cases postponed as some immigration courts are limiting dockets and others are closing. Refugee resettlement is temporarily suspended as well, according to the United Nations, because of the Coronavirus pandemic.

There are about 100 confirmed cases of COVID-19 in Mexico, and about 600 in Canada, compared to nearly 8,000 cases in the United States.

Immigrant advocates accuse the Trump administration of exploiting the pandemic to advance its crackdown on asylum-seekers, and the partial closure of the southern border where thousands have amassed in Mexico is sure to draw legal challenges.

"President Trump has been falsely scapegoating immigrant communities in the name of public safety since he came into office," said Michelle Brané at the Women's Refugee Commission. "This rule would unquestionably violate both domestic and international law and is an abdication of our moral responsibility to protect vulnerable people."

In addition, immigrant advocates and physicians who study detention facilities are calling on U.S. authorities to release detained immigrants who pose no threat to public safety — starting with detainees who have medical conditions that put them at high risk for severe complications from COVID-19.

In immigration detention, there's no such thing as social distancing. Detainees often live together, in large rooms with rows of cots. Immigrant advocates say that makes detention centers ideal breeding grounds for the virus.

"It is a disaster that is waiting to happen," said Eunice Cho, a lawyer with the American Civil Liberties Union, which filed a lawsuit this week seeking the release of immigrants with a range of medical problems who are detained by Immigration and Customs Enforcement in Tacoma, Wash.

"Medical conditions in ICE detention facilities are inadequate in normal times," Cho said. "These are not normal times, and ICE needs to take stock of that."

ICE is currently holding more than 37,000 immigrants in its detention centers. Nearly half have been accused of no crime other than civil immigration violations, and many probably would not have been held under previous administrations.

ICE is scaling back its enforcement activities. In a statement, the agency says it will stop making arrests except in "mission critical" criminal cases. The agency says it will focus on individuals who pose a public safety risk and delay other enforcement actions.

Court cases involving ICE detainees will continue, despite calls to suspend all hearings from the union representing immigration judges, prosecutors and immigration attorneys. Hearings for immigrants who are not in detention have been postponed until April. Immigration courts in New York, Atlanta, Houston and elsewhere will close.

In a statement, ICE says it is taking precautions to protect detainees. The agency has temporarily suspended social visits at all of its detention centers. As of Tuesday, ICE says, there were no confirmed cases of COVID-19 in its detention facilities.

But in New Jersey, one staffer at the Elizabeth Detention Center has voluntarily self-quarantined after feeling ill, and has been tested for the COVID-19.

"It would seem to be only a matter of time before the virus makes its way into these particular settings," said Anwen Hughes, an attorney with the nonprofit Human Rights First who represents immigrants being held in Elizabeth.

"There's the opposite of social distancing" at the facility, Hughes said. "It's like social piling on."

It's not just immigrant advocates who want to see ICE detainees released.

"A lot of people think that, well, these are, you know, controlled locked facilities and you just lock them up and seal them off and the virus isn't going to get in. Well, it's not that simple," said Dr. Josiah Rich, an epidemiologist at Brown University who studies infectious disease in correctional facilities.

It's not just the detainees who are at risk if there's an outbreak, Rich says, because an influx of sick detainees could overwhelm local medical facilities.

"So you'll have a whole group of people that are going to be infected because of rapid spread in that correctional and detention facility," he said. "And then all of sudden, those people will get sick all at once."