“Congress must get together and immediately eliminate the loopholes at the border!” Trump tweeted. “If no action, border, or large sections of border, will close. This is a national emergency!”
Trump has repeatedly threatened to close the US southern border, despite pressure from companies worried that a shutdown would hurt supply chains and $1.7bn in daily trade.
Last week, he said that closure could come as early as this week unless Mexico stopped people from crossing the border irregularly. He appeared to walk back on that threat, however, saying he was pleased with steps Mexico has taken in recent days.
On Tuesday, Senate Majority Leader Mitch McConnell told NBC News that closing the border “would have potentially catastrophic economic impact”.
He added that although he agrees with Trump that there is a “border crisis”, he would hope that the US would not close the border.
A transfer of US border agents to immigration duties has slowed commercial traffic at three crossings, with gridlock in El Paso extending for hours.
US Customs and Border Protection (CBP) said on Tuesday it would suspend cargo operations every Saturday at one of its crossing points in El Paso until it has enough staff to operate fully, Mexican media reported.
On Wednesday, some, but not all, lanes were open to commercial traffic at El Paso, Laredo and Otay Mesa. The longest wait stretched up to three hours at a section of the El Paso crossing, according to CBP. In Ciudad Juarez, across the border from El Paso, lines of trucks were longer than usual, according to a Reuters witness.
Mexican President Andres Manuel Lopez Obrador said on Wednesday there were no “serious problems” at the US-Mexico border after commercial traffic slowed at several crossings.
Mexico’s Foreign Minister Marcelo Ebrard is in constant communication with US authorities to avoid conflict and to keep the border open, Lopez Obrador told reporters at his regular morning news conference.
“It’s not in anyone’s interest to close the border,” he said.
Trump declared a national emergency in February to circumvent Congress in obtaining billions of dollars to build his promised wall on the US-Mexico border.
Congress passed a bill to revoke the order, but Trump vetoed the measure. The emergency is also being challenged in the courts.
Thousands of people fleeing Honduras, El Salvador, Guatemala and elsewhere have made their way to the border in recent months with the hopes of applying for asylum.
Dozens have told Al Jazeera they are fleeing political persecution, violence and extreme poverty.
Last week, US government officials said March was on track to have the highest number of monthly border crossings in over a decade, with more than 100,000 apprehensions and encounters of people deemed inadmissible at US ports of entry. They added there has been a surge in the number of family units crossing the border.
Hundreds of migrants are being bussed further inland and dropped off at bus stations and churches. Trump has mocked and vowed to end that so-called “catch and release” practice in the past.
But Immigration Customs and Enforcement (ICE) officials have been releasing asylum-seeking families so quickly that they don’t even have time to make travel arrangements. Families are given court dates, a head of household is often fitted with an ankle monitor, and they are dropped off at a charity-run shelter or bus station.
San Antonio received part of that surge in recent days, forcing the city to open a help centre with food for migrants.
In El Paso, where shelters and churches are at capacity and seats on buses headed out of the city are getting harder to find, authorities briefly resorted to holding migrants in a pen lined with concertina wire under the shade of a bridge that connects the American city to Juarez, Mexico. They closed the makeshift holding area over the weekend and moved the migrants to a place with more shelter.
“They treated us like animals,” said Herling Jerlyn, a teenager from Guatemala.
About 22,000 immigrants have been released in Arizona in the past three months. In the Phoenix area, the nonprofit organisations and churches taking them in have a capacity of only 700 a week, said Connie Phillips, president and CEO of Lutheran Social Services in the Southwest.
That means immigration authorities have to drop off families by the busload at places not designed to take them in, such as the Greyhound station in Phoenix.
US authorities have also ramped up the controversial policy of returning migrants back to Mexico while they wait out their asylum claims. Rights groups have said the practice violates human rights. The policy is being challenged in the courts.