US President Donald Trump will not sign a stopgap spending bill because it does not contain border wall funding, Republican lawmakers said Thursday, dramatically escalating chances of a government shutdown before Christmas.

Trump's move appears to be a hardening of his demand for $5 billion in funding for the wall on the US-Mexico border, something he has fought for since he began campaigning for president in 2015.

"The president informed us that he will not sign the bill that came over from the Senate last evening because of his legitimate concerns for border security," outgoing House Speaker Paul Ryan said after he and other Republicans met with Trump to try to come to a deal before the holidays.

"So what we're going to do is go back to the House and work with our members," Ryan added. "We want to keep the government open, but we also want to see an agreement that protects the border."

Trump's rejection of a bill that would keep the government fully operational through February 8 comes just one day before funding expires for key agencies -- sending lawmakers scrambling for a new compromise.

So far, Democrats have stood firm, saying they will not support a spending measure that funds Trump's wall.

"In terms of wall funding, that's a non-starter," said top Democrat Nancy Pelosi, Ryan's likely successor as speaker when Democrats reclaim the majority on January 3. "I think they know that."

Fears of a shutdown, adding to other concerns, sent US stocks tumbling, with the Dow shedding about two percent.

Trump's startling about-face on the deal came a day after the US Senate unanimously passed a stopgap measure that would extend funding for six weeks.

Trump had backed off his shutdown threat earlier this week, but it roared back to life as he accused Democrats of "putting politics over country" by not supporting his call for a wall, which he insists will curb illegal immigration.

"What they are just beginning to realize," Trump tweeted about opposition Democrats, "is that I will not sign any of their legislation, including infrastructure, unless it has perfect Border Security."

Trump's move may have been influenced by virulent criticism from far-right lawmakers, some of whom have publicly called on the Republican president to stick to his guns on wall funding.

- 'Meltdown' -
Members of the conservative House Freedom Caucus took to the House floor late Wednesday, imploring Trump to stand firm.

"Mr. President, we'll back you up," caucus chairman Mark Meadows said in the chamber. "If you veto this bill, we'll be there."

Meadows was among the Republicans who met with Trump at the White House Thursday.

White House spokeswoman Sarah Sanders said Trump "is continuing to weigh his options" on the path forward.

"At this moment, the president does not want to go further without border security, which includes steel slats or a wall," she said.

Pelosi meanwhile accused Republicans of having a "meltdown" over whether to pass the stopgap measure or force a shutdown.

House Majority Leader Kevin McCarthy expressed optimism about reaching a new compromise before lawmakers leave Washington for the holidays.

"We believe there's still time," he said.

But some Democrats expressed frustration that one of Ryan's final acts as speaker was to serve Trump rather than put the Senate bill on the House floor, where it would likely pass with bipartisan support.

That would force Trump to actually veto the measure.

"Why not allow a vote on a bipartisan Senate bill and let democracy work in the House?" Senator Chris Van Hollen said. "This is a cowardly way to govern."

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© Agence France-Presse