WASHINGTON (REUTERS) - US President Donald Trump on Thursday (Jan 10) threatened to use emergency powers to bypass Congress and get billions of dollars to pay for a wall on the US-Mexico border as a partial US government shutdown over the issue stretched into its 20th day.

Trump flew to the Texas border with Mexico to try to bolster his case for the wall, flanked by tearful family members of people killed by illegal immigrants and border patrol agents who are not receiving pay checks during the shutdown.

"If we don't have a barrier, a very substantial barrier of some kind, you're not going to be able to solve this problem," Trump said at a briefing where plastic-wrapped bricks of heroin, seized guns and a plastic bag full of cash were prominently displayed.

Trump is adamant that a government funding Bill to end the shutdown include US$5.7 billion (S$7.7 billion) for a border barrier - his signature campaign promise.

The standoff has left a quarter of the federal government closed down and hundreds of thousands of federal employees without pay.

A day after he stormed out of a meeting with Democratic leaders that was aimed at negotiating an end to the funding standoff, Trump attacked them for refusing his demand, calling them harder to deal with than China, a rival power.

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

Trump signalled he was prepared for the showdown to stretch into late January, cancelling plans to attend the World Economic Forum in Davos, Switzerland, which is scheduled to start on Jan 22.

'HIS ONLY WAY OUT'

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," Trump told reporters at the White House.

"I'm not prepared to do that yet, but if I have to, I will."

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

"I would almost say definitely," he said.

The declaration would circumvent Congress' power over the national purse strings, giving 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.

Even some Republicans who want to build a wall have said they do not want money to taken from the military for it and others see it as an overreach of executive powers.

Senator Joe Manchin, a moderate Democrat who has cultivated good relations with Trump, said a national emergency declaration by Trump would be "wrong but I think that's his only way out" of the government shutdown impasse.

If Trump were to make such a declaration, Manchin predicted the Senate would immediately pass legislation to fund the federal agencies that have been partially closed.

"If he did it tonight, everybody would be right here to vote," Manchin told reporters.

NO TALKS UNDER WAY

Republican Senator Roy Blunt told reporters that he knew of no discussions now under way to end the border wall fight that has led to the shutdown.

Pelosi told reporters Trump keeps "increasing the obstacles to finding a solution" to the shutdown.

"He loves the distraction that this is from his other problems," she said at a briefing.

Pressure on both sides could intensify on Friday when about 800,000 federal employees - including border patrol agents and airport security screeners - miss their first pay cheques.

About half of the workers who are deemed essential to national security - people such as prison guards and airport employees - are working without pay. Others are staying home on furlough.

At the FBI, where most agents continue to work, concerns are building that operational funds the bureau needs to conduct investigations, including sensitive undercover operations, are beginning to dry up, said Tom O'Connor, president of the FBI Agents Association.

The lack of pay cheques also could threaten the financial security of FBI personnel and conceivably raise questions about their security clearances, O'Connor said.

The shutdown, which began on Dec 22, will be the longest in US history if it is still under way on Saturday.

ACROSS THE RIO GRANDE

Trump says undocumented immigrants and illegal drugs are streaming across the border from Mexico, despite statistics that show illegal immigration there is at a 20-year low and that many drug shipments likely are smuggled through legal ports of entry.

Just across the Rio Grande river from McAllen, in the Mexican city of Reynosa, a dozen recent deportees and US-bound migrants gathered near a cross that memorialised dead migrants.

Plagued by kidnappings, Reynosa is so dangerous for migrants that locals advise new arrivals against any trip alone out of the two migrant shelters.

"Building a wall for a country, a nation so big, for a border so immense, so many kilometers is ridiculous," said Jose Ramon Poso Briseno, a Honduran man who has waited in Reynosa's Senda de Vida shelter for five months but has so far balked at crossing the river out of fear of cartel violence.