The Biden administration is ramping up exceptions to a public health order that has largely shut the U.S.-Mexico border to migrant traffic since last year because of the pandemic.

More migrants are being granted humanitarian exceptions because they are considered the most vulnerable, including families with young children and transgender people who had been living in dangerous conditions in Mexican border towns.

This comes as the number of migrants apprehended at the southern border topped 170,000 in April for the second consecutive month. The majority are still being turned away, but an increasing number of single adults and families are being allowed into the U.S. to seek asylum.

"We are working to streamline a system for identifying and lawfully processing particularly vulnerable individuals who warrant humanitarian exceptions under the order," Homeland Security Department spokeswoman Sarah Peck said in a statement to NPR, referring to the health order aimed at stopping the spread of COVID-19.

"This humanitarian exception process involves close coordination with international and non-governmental organizations in Mexico and COVID-19 testing before those identified through this process are allowed to enter the country," Peck said.

But migration experts and aid groups say the Biden administration hasn't explained how the system works, creating widespread confusion in border communities about who is considered vulnerable.

"There's no clear set of criteria for which families are allowed in," said Jessica Bolter, an analyst at the Migration Policy Institute in Washington, D.C. "So it can really seem to migrants kind of like a game of chance."

The Biden administration has not sought to publicize this streamlined system of humanitarian exceptions to the health order. Instead, President Biden and top officials have repeatedly urged Central American migrants not to make the dangerous journey north because the border remains largely closed.

Immigrant advocates say administration officials are worried about touching off another surge of migration.

The number of migrants apprehended after crossing the southern border soared to a 20-year high in March, and rose slightly again in April, according to official numbers released Tuesday.

Nonprofits and immigrant advocates describe a patchwork of ways that asylum seekers are now being allowed into the U.S. — after being barred from entry for months under the pandemic health order from Centers for Disease Control and Prevention known as Title 42.

Under one arrangement, the Biden administration is working with a consortium of non-governmental organizations to identify the most vulnerable migrants so they can be allowed to enter the U.S., according to two people who are familiar with the process.

That system began last week on a small scale in El Paso, Texas, and is expected to be rolled out across the Southern border. Migrants are screened in Mexico before they present themselves to U.S. Customs and Border Protection officials at ports of entry.

Ruben Garcia, executive director of Annunciation House, a nonprofit in El Paso, Texas, says he's heard that migrants are gaining entry by establishing they are vulnerable in Ciudad Juárez, Mexico, just across the border. His organization provides temporary shelter to migrants.

"What exactly is the criteria, who chooses them; that I can't tell you," Garcia said. "I just don't know."

Under these Title 42 exceptions, groups of up to 50 people are being allowed to cross from Juárez to El Paso almost daily, Garcia said. Most other migrants are still being expelled.

"Here in El Paso, Title 42 is still being enforced," Garcia said.

Nearly two-thirds of migrants crossing in families — about 32,500 people in April alone — and a growing percentage of single adults were allowed into the U.S. to pursue asylum claims last month.

Many are being allowed in because parts of Mexico are refusing to take back families with small children, administration officials said.

It's not known how many migrants were granted humanitarian exceptions in April. But the number of migrants allowed to cross after presenting themselves at ports of entry — rather than crossing illegally — more than doubled to nearly 1,800.

Transgender migrants also have been allowed recently to cross through ports of entry to be processed by U.S. immigration authorities and released to reunite with family or sponsors while their asylum cases move through immigration court.

A transgender migrant shelter in Juárez is expected to close this week. Alexa Ponce, 25, had been living there for more than a year after arriving from El Salvador just before the border pandemic restrictions took effect in March.

She's scheduled cross into the U.S. Wednesday with a group of seven other transgender women to request asylum.

"I feel an avalanche of emotions," said Ponce, who dreams of attending college. "I'm very happy, but I'm also nervous. I'm not sure how things will go for me. I want to work and have a more dignified life than I could in my country."

In San Diego, immigrant advocates report at least five migrant families are granted humanitarian exceptions each day. Al Otro Lado, Jewish Family Service and other non-governmental organizations have been petitioning CBP to allow them into the U.S.

These groups visit la migrant encampment just across the border in Tijuana, Mexico,as well as local shelters, to help identify especially vulnerable migrants.

They advise families who are deciding between waiting to be allowed to cross together at a port of entry and "self-separating" by sending their children across the border alone. The Biden administration already grants entry to most unaccompanied migrant children.

They also accompany families who are allowed to cross to the border. Last week, a case manager with the legal aid group Al Otro Lado, which is based in Tijuana, waited with several families at the port of entry, communicating with CBP agents there.

One of the families — Valeria and her two children — had been stuck in Tijuana since leaving Michoacán, Mexico, last year, fleeing domestic abuse. She asked that NPR not use her full name because she's still worried the violence will follow her.

"I feel safe right now, because I'm on the way to be with my family," she said, before she crossed the border. Her father and sister already live in the United States. "In Tijuana, there's fear, but we're on the way to safety."

The Biden administration has come under fire from Republicans, who blame a recent surge in the number of migrants arriving at the border on President Biden's decision to roll back some of his predecessor's hardline immigration policies.

But the White House is also under pressure from immigrants' rights groups, who say the administration should be doing more to allow vulnerable migrants to seek asylum.

They're calling on the administration to lift the Title 42 public health order, which was put in place last year under former President Trump, effectively closing the southern border to asylum seekers.

"A process to immediately help particularly vulnerable families is helpful, but it is not a substitute for completely ending the Trump administration's Title 42 policy," said Lee Gelernt, a lawyer at the American Civil Liberties Union, which has challenged the order as it applies to migrant families in court.

"The policy is unlawful and inhumane," Gelernt said.

The ACLU agreed to put its lawsuit on hold in February, and negotiated its own exception process with the Biden administration, according to Gelernt. Since late March, he said the ACLU has been referring up to 35 vulnerable families per day for entry into the U.S.

But the administration defends its use of the Trump-era public health order, and says it's up to the CDC to rescind it.

"We continue to expel single adults and families under Title 42," Homeland Security Secretary Alejandro Mayorkas in an interview with NPR's All Things Considered last week. "That is the province of the CDC to assess the public health needs of the situation with respect to the pandemic."

Mayorkas sought to downplay the scope of exceptions that the administration is granting.

"We do make exceptions for discrete humanitarian reasons to address particular vulnerabilities, and we have done so from the very outset," he told NPR.

"The border is not open," he said.