President Trump has threatened to close the US-Mexico border as cars lined up at the border waiting to cross into the United States.

The Trump administration on Sunday doubled down on its threat to shut down the southern border with Mexico, a day after it cut aid to Central American countries that President Donald Trump accused of deliberately sending migrants to the United States.

In a tweet later that same day, Trump blamed Democrats for what he described as 'a ridiculous asylum system' that has 'major loopholes' and accused Mexico of not helping with migration.

At the Cordova-Americas international bridge in Ciudad Juarez, Chihuahua state, a long line of cars could be seen queuing to cross the border over the weekend after lanes were partially closed.

Trump tweeted yesterday afternoon: 'The Democrats are allowing a ridiculous asylum system and major loopholes to remain as a mainstay of our immigration system. Mexico is likewise doing NOTHING, a very bad combination for our Country. Homeland Security is being sooo very nice, but not for long!'

vehicles queued to cross the Cordova-Americas international border crossing bridge in Ciudad Juarez, Mexico, on Friday

U.S. Customs and Border Protection (CBP) agents stand guard at the Cordova-Americas international bridge in El Paso, Texas, U.S. which lanes remain partially closed delaying the crossing of vehicles in Ciudad Juarez, Chihuahua state, Mexico on Sunday

Customs and Border Protection (CBP) agents in El Paso, Texas, partially closed the lanes on the Cordova-Americas international bridge, delaying the crossing of vehicles on Sunday

Faced with a surge of asylum seekers from Central American countries who travel through Mexico, Trump said on Friday there was a 'good likelihood' he would close the border this coming week if Mexico did not stop unauthorized immigrants from reaching the United States.

Without providing evidence, he also accused the nations of having 'set up' migrant caravans and sending them north.

Closing the border could disrupt millions of legal crossings and billions of dollars in trade.

Customs and Border Protection (CBP) agents in El Paso, Texas, partially closed the lanes of the bridge crossing, delaying vehicles getting into the US.

Trump on Friday again accused Mexico of failing to curb the flow of migrants illegally entering the US, and threatened to close the border 'next week' unless something changes.

The security measure is due to the 'humanitarian crisis' declared by CBP because it has processed 13,400 migrants detained by the Border Patrol.

Speaking to ABC's 'This Week' show, White House acting Chief of Staff Mick Mulvaney said the president had few other options in the absence of any support from Democrats for more border security or legislative action to change the immigration law.

A U.S. Border and Customs and Border Protection (CBP) official stands next a concrete K-rail with concertina wire at Cordova-Americas bridge to enhance security efforts in preparation for increasing number of migrants arriving at the border

Trump (pictured with first lady Melania Trump walking off Marine One at the White House yesterday) threatened to close the border with Mexico

Marlene Ruiz, a Cuban who is hoping to apply for asylum, is pictured on her knees in front of border agents on an international bridge at the border between Mexico and the U.S.

'Faced with those limitations, the president will do everything he can. If closing the ports of entry means that, that's exactly what he intends to do,' Mulvaney said. 'We need border security and we're going to do the best we can with what we have.'

White House adviser Kellyanne Conway told Fox News Sunday that the situation at the border was at 'melting point' and said the president was serious in his threat. 'It certainly is not a bluff. You can take the president seriously.'

Neither Trump aide offered any specific details or timeline for the potential border shutdown.

Trump blamed Democrats for what he described as 'a ridiculous asylum system and major loopholes to remain as a mainstay of our immigration system'.

'Mexico is likewise doing NOTHING, a very bad combination for our Country. Homeland Security is being sooo very nice, but not for long!' he added in his tweet, without explaining what he meant.

In a letter to Congress on Thursday, Homeland Security Secretary Kirstjen Nielsen said she would soon propose legislative language to allow her agency to return unaccompanied immigrant children home, to keep migrant families in custody together during the immigration process and to allow people seeking asylum to make their applications from Central America.

A group of Cubans hoping to apply for asylum wait in front of border agents on an international bridge at the border between Mexico and the U.S. in in Ciudad Juarez

U.S. Customs and Border Protection (CBP) agents stand guard at the Cordova-Americas international bridge in El Paso, Texas which lanes remain partially closed delaying the crossing of vehicles in Ciudad Juarez, Chihuahua state, Mexico

Asked about migration in general and Trump's threat to shut the border specifically, Pope Francis told reporters on Sunday as he returned from Morocco on Sunday that 'builders of walls, be they made of razor wire or bricks, will end up becoming prisoners of the walls they build.' He did not mention Trump in his response.

At a Saturday rally on the border in El Paso, Texas, Democratic presidential hopeful Beto O'Rourke denounced Trump's immigration policies as the politics of 'fear and division.'

Trump has repeatedly said during his two years in office that he would close the U.S. border with Mexico.

His latest threat had workers and students who frequently cross the border worried about the potential disruption to their lives.

Mexico is the largest importer of U.S. exports of refined fuels like diesel and gasoline, some of which move by rail. It is unclear if rail terminals would be affected by closures.

'It would have a widespread and dramatic impact on our markets almost immediately,' said an executive at a company that ships products by rail to Mexico, citing its dependence on U.S. natural gas, propane, gasoline and ultra-low-sulfur diesel fuel.

The U.S. government says it is struggling to deal with a surge of asylum seekers from countries in Central America who travel through Mexico. On Saturday, it cut aid to El Salvador, Guatemala and Honduras - a move Democrats warned would only worsen the situation.

Detained child migrants waiting to be transported by the U.S. Border Patrol at the border of the United States and Mexico yesterday

Border Patrol agents load detained migrants into a van at the border of the United States and Mexico yesterday

'What we need to do is focus on what's happening in Central America, where three countries are disassembling before our eyes and people are desperately coming to the United States. The president's cutting off aid to these countries will not solve that problem,' the number two Senate Democrat, Dick Durbin, told NBC News' Meet the Press.

He also cast doubt on the viability of shutting the border, describing the threat as 'totally unrealistic'.

March is on track for 100,000 border apprehensions, Department of Homeland Security officials said, which would be the highest monthly number in more than a decade.

Most of those people can remain in the United States while their asylum claims are processed, which can take years because of ballooning immigration court backlogs.

Trump said on Friday he planned to hold a media event at the border in the next two weeks. According to his public schedule, the president will visit Calexico, California, on Friday.

While some Trump critics say he is bluffing, closing the southern border is not unprecedented.

Presidents Richard Nixon and Ronald Reagan both shut the border over drug-related issues, while President Lyndon Johnson closed it briefly after the assassination of John F. Kennedy in 1963.