US President Donald Trump has ramped up his border wall campaign by scrapping plans to attend the annual World Economic Forum in Switzerland, threatening to launch a national emergency and pouring billions of his own money to begin its construction. 

Mr Trump met with Border Patrol agents in Texas on Friday (Thursday local time) before visiting the US-Mexico border in an effort to bolster his campaign for $5.7 billion ($7.9 billion) in wall funding.

It comes a day after he stormed out of a meeting with Congressional leaders aimed at finding a deal to end the funding stand-off. 

The US has entered day 20 of Mr Trump’s partial government shut down, leaving about 800,000 government employees in the lurch since December 21 without almost three weeks of pay.

It is on the verge of becoming the longest government shutdown in US history, beating out former Democratic President Bill Clinton’s 21-day shutdown of 1995-96.

The president told reporters that he had intended to speak at the World Economic Forum later this month but would not attend if the shutdown continued.

The cancellation quashes any opportunity for Mr Trump to meet with other world leaders about economic issues, including trade.

During his meeting with border patrol agents, Mr Trump stood behind a display of stacks of smuggled firearms, illegal drugs and money seized at the point of entry, while he preached about the need to build a wall along the Mexican border.

President Trump speaks during his visit to US Border Patrol in McAllen, Texas. Photo: Getty

He blamed previous presidents for their alleged inaction along the southern border.

“I’ve seen all the speeches made by all the presidents and a lot of people that worked under the presidents and you say, ‘What happened?'” he said. “Nothing happened.”

He claimed there had never in history been so many apprehensions at the southern border. However US Customs and Border Patrol data say the exact opposite is happening, with the number of people illegally crossing the border actually at historic lows.

During the round-table discussion about border security and immigration, Mr Trump praised border patrol agents as “heroes” who were “highly respected in our country”.

President Trump meets with Border Patrol agents in McAllen, Texas, and thanks them for their tireless work to keep our country safe: pic.twitter.com/sDHmkcEgAi

— The White House 45 Archived (@WhiteHouse45) January 10, 2019

Before departing to Texas, Mr Trump told reporters Mexico is already “paying for the wall indirectly” through the “really great trade deal we just made”.

“When I said Mexico will pay for the wall in front of thousands and thousands of people, obviously they’re not going to write a cheque.”

He also attacked the Democratic congressional leaders who oppose his funding demand.

“I find China, frankly, in many ways to be far more honourable than crying Chuck and Nancy. I really do,” Mr Trump said on Thursday, referring to House of Representatives Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer.

“I think that China is actually much easier to deal with than the opposition party.”

During the press conference, several hundred protesters chanted and waving signs opposing a border wall.

Donald Trump's visit to push for wall funding draws cheers and protests in Texas. https://t.co/wvcCoyMqwI pic.twitter.com/k4s6z5J1MX

— Texas Tribune (@TexasTribune) January 10, 2019

Mr Trump insists a government funding bill to end the shutdown must include the $US5.7 billion for a border barrier, his signature campaign promise.

After the meeting, Mr Schumer accused the Republican president of “temper tantrum” tactics and Ms Pelosi called Mr Trump “petulant”.

Mr Trump said his lawyers had told him he had the power to invoke national emergency powers to get his wall funded, a course of action that Democrats have said may be illegal.

“I have the absolute right to declare a national emergency,” Mr Trump told reporters as he left the White House.

“I’m not prepared to do that yet, but if I have to, I will.”

🚨LINDSEY GRAHAM CALLS FOR TRUMP TO USE EMERGENCY POWERS FOR WALL 🚨

“It is time for President Trump to use emergency powers to fund the construction of a border wall/barrier. pic.twitter.com/eVXuArgdli

— Benny Johnson (@bennyjohnson) January 10, 2019

If Congress fails to bend on the border wall funding, Mr Trump said he probably would declare a national emergency.

“I would almost say definitely,” he said.

The declaration would circumvent Congress’s power over the national purse strings, giving Mr Trump the ability to redirect money from the Department of Defence to his proposed wall.

Such a step likely would prompt an immediate legal challenge over constitutional powers from congressional Democrats.

A subsequent court fight could be protracted, making room for the shutdown to be ended in the interim.

The final outcome would then be left up to judges, not the president and Congress.

-with AAP