Migrant children and families are dangerously packed into holding facilities on the southwest border, lawmakers and child­-welfare monitors warned Friday, as Customs and Border Protection weighed taking the emergency step of putting migrant families on airplanes to states near the Canadian border for processing.

The strain of a sudden, sharp spike in apprehensions became clear as Department of Homeland Security officials and Democratic and Republican lawmakers toured the El Paso sector of the border and saw hundreds of children packed into large, open rooms and families streaming across the border at night.

Conditions were even worse hundreds of miles to the southeast in the Rio Grande Valley, a court-appointed monitor told a federal judge Friday, saying the crowding in Border Patrol facilities was “profound,” “not sustainable” and at risk of unraveling.

The Biden administration is rushing to manage a rapidly growing influx at the border even as it plays down the emergency as a “challenge,” not a crisis. But a growing number of Republicans and some Democrats say the situation is spiraling out of control, with thousands of unaccompanied minors filling shelters and thousands more in border holding areas not meant for children.

Health and Human Services said it had 9,800 unaccompanied minors in shelters Friday, but so many new groups were fast arriving that 4,700 more children and teenagers were waiting in Border Patrol facilities designed for adults. Their average time in custody was 135 hours, nearly twice the legal limit.

CBP agents in Texas’s Rio Grande Valley, where the largest groups are arriving, reported that groups of families and children as large as 400 have arrived in the past 24 hours.

Brian Hastings, the Border Patrol chief in the Rio Grande Valley, said in a tweet that agents took more than 2,000 into custody Thursday, with 10,000 arrests and detentions in less than a week. The busy sector has taken in more than 34,000 migrants so far in March, he wrote. That figure is already twice as high as the entire month of January, records show.

Internal DHS communications reviewed by The Washington Post signaled that overcrowding at border facilities has become a life-or-death emergency and urged HHS to immediately take custody of large numbers of minors.

Paul Wise, a court-appointed monitor who reported on conditions for migrant children as part of a long-running lawsuit Friday, told U.S. District Judge Dolly Gee in Los Angeles that he witnessed “significant overcrowding” in facilities in Donna, Tex., and Border Patrol stations last week. He said he also visited emergency intake sites in Midland, Tex., and Dallas on Friday.

He said the major issue is CBP’s “inability” to quickly move the unaccompanied minors to HHS shelters.

“The current status does not represent a sustainable situation,” Wise said, adding that if the influx continues to increase, the current systems “will inevitably begin to unravel.”

Homeland Security Secretary Alejandro Mayorkas had no immediate comment Friday as he joined a bipartisan delegation of senators from the Senate Homeland Security and Governmental Affairs Committee in El Paso. DHS said it planned to view operations and receive a briefing on the processing, shelter and transfer of unaccompanied children at the border.

Senators from both parties described dire conditions after their visits.

Sen. Rob Portman (R-Ohio), said he saw migrants streaming over the border during a night patrol Thursday in El Paso. Sen. Chris Murphy (D-Conn.) said hundreds of children were “packed into big open rooms” and that he “fought back tears” as he watched a sobbing 13-year-old girl explain through a translator that she was terrified after having been separated from her grandmother.

Sen. Shelley Moore Capito (R-W.V.) called the surge of minors “a tragedy.”

“Swift action needs to be taken NOW to end this humanitarian crisis,” she said on Twitter.

Republicans and Mayorkas have sparred over whether the current influx represents a “crisis,” a term the Obama administration used in 2014 but that Biden administration officials have avoided.

A new spike in the number of families and children crossing the Rio Grande into South Texas on Friday forced CBP to request airplanes to transport migrants to states near the Canadian border for processing, according to two Department of Homeland Security officials and an email reviewed by The Post.

The internal communications do not indicate which facilities.

CBP said in a statement Friday night that “currently there are no plans to transfer migrants from the Southwest border to the Northern or Coastal borders.” Officials did not respond to questions about whether the government would furnish winter clothing to migrants sent to states such as Montana, North Dakota and Michigan, where the Border Patrol has far fewer staff members and generally smaller facilities.

Republicans have slammed the Biden administration for unwinding some of the Trump administration’s border clampdowns, including phasing out a program that required migrants to await their asylum hearings in Mexico and halting expulsions of unaccompanied minors under a public health order Trump imposed in March, citing the pandemic.

More than 29,000 unaccompanied minors crossed into the United States from October through last month, nearly double the number from the same period last year, joining a growing number of migrant families and single adults.

The Biden administration instead has expanded capacity at shelters and emergency intake centers, surged medical teams and DHS to the border, and dispatched the Federal Emergency Management Agency.

Officials also have discouraged migrants from coming to the border illegally, saying the Trump administration had “gutted” the asylum system, and they needed time to rebuild.

But migrants from Central America and elsewhere have continued to arrive, propelled by the devastation and hunger that followed a pair of powerful hurricanes that hit Honduras and Guatemala late last year, as well as ongoing violence and poverty.

Portman, who announced in January that he will not seek a third term, citing his frustration with “partisan gridlock,” said border agents told him it was difficult to retain personnel because of large numbers of migrants and drugs crossing into the United States.

He called the situation a “crisis” and said President Biden’s approach so far has “failed.”

“It was stunning to see how many people, including unaccompanied children, unlawfully crossed the border during my ride-along last night,” said Portman, the ranking Republican on the Homeland Security Committee.

He said smugglers are using the families to distract agents from trafficking methamphetamines and other drugs into the United States.

“We are still early in the year and not yet in the peak season for migrant travel, so we are likely to see continued increases in unlawful immigrants and deadly narcotics at our border unless the Biden administration changes its failed approach,” he said.

But Murphy, the Democrat, said he did not fault the Biden administration for the current crisis and said it is scrambling to rebuild a system that the Trump administration left in tatters.

“The Biden Administration is trying their best to uphold the rule of law with humanity. They have a ton of work ahead to clean up the mess Trump left them, but their intentions are true,” Murphy said on Twitter. “The desperation these kids and families are fleeing is hard to describe. The memory of that 13 yr old girl will be [with] me forever.

“So long as conditions are abysmal in places south, people will find a way to get here, no matter how high the wall is or how many border agents,” he said. “Seeing the wall in person is unexpectedly devastating. How did it come to this, that a country defined by our warm embrace of immigrants now must be defined by our irrational fear of them? We must be better.”

Correction: An earlier version of this story said officials did not respond to questions about whether the government would furnish winter clothing to migrants sent to states such as Montana, Michigan and South Dakota. It should have said North Dakota.

Seung Min Kim, John Wagner and Colby Itkowitz contributed to this report.