The US government has shut down much of its operations, setting off what might be a long, gruelling standoff that could lead to the loss of thousands of federal jobs.

Deep partisan divisions prevented Congress and the White House from reaching an 11^th-hour funding deal that would have broken the budget deadlock and avoided the shutdown, which began at 12.01am Wednesday (local time).

The White House website now features a ticking clock showing how long the shutdown has been going for, with the caption “Democrats have shut down the government”.

With no clear path out of the impasse, agencies have warned that the 15th government shutdown since 1981 will halt the release of a closely watched September employment report, slow air travel, suspend scientific research, withhold pay from US troops and lead to the furlough of 750,000 federal workers at a daily cost of $US400 million ($A600 million).

Trump, whose campaign to radically reshape the federal government is already on track to push out some 300,000 workers by December, had earlier warned congressional Democrats a shutdown could clear the path for “irreversible” actions.

“We can do things during the shutdown that are irreversible, that are bad for them and irreversible by them, like cutting vast numbers of people out, cutting things that they like, cutting programs that they like,” he told reporters from the Oval Office.

The shutdown began hours after the Senate rejected a short-term spending measure that would have kept government operations afloat through November 21.

Democrats opposed the legislation over Republicans’ refusal to attach an extension of health benefits for millions of Americans that are due to expire at the end of the year. Republicans say the issue must be addressed separately.

At issue on the government funding front is $US1.7 trillion ($A2.6 trillion) for agency operations, which amounts to roughly one-quarter of the government’s total $US7 trillion ($A10.6 trillion) budget. Much of the remainder goes to health and retirement programs and interest payments on the growing $US37.5 trillion ($A57 trillion) debt.

Independent analysts warn the shutdown could last longer than the budget-related closures of the past, with Trump and White House officials threatening to punish Democrats with cuts to government programs and the federal payroll.

Trump budget director Russell Vought, who has called for “less bipartisan” appropriations, threatened permanent layoffs last week in the event of a shutdown.

The longest government shutdown in US history stretched over 35 days during December 2018 and January 2019 during Trump’s first term in office, in a dispute over border security.

“All they want to do is try to bully us. And they’re not going to succeed,” Senate Democratic leader Chuck Schumer said in a floor speech a day after a White House meeting with Trump and other congressional leaders that ended with the two parties far apart.

Trump’s Republicans hold majorities in both chambers of Congress, but legislative rules require 60 of the 100 senators to agree on spending legislation. That means that at least seven Democrats are needed to pass a funding bill.

University of Chicago professor Robert Pape said the unusually polarised US political climate in the aftermath of conservative activist Charlie Kirk’s assassination and the growing power on the extreme wings of both parties could make it harder for party leaders to agree on a deal to reopen the government.

“The rules of politics are radically changing and we can’t know for sure where all of this is going to end,” said Pape, who studies political violence.

“Each side would have to backtrack against tens of millions of truly aggressive supporters, their own constituents, which is going to be really hard for them to do,” he said.

Before the shutdown, Trump reached out to his own supporters with a deepfake video showing manipulated images of Schumer appearing to criticise Democrats while top House Democrat Hakeem Jeffries stood next to him, with a crudely drawn sombrero and moustache imposed over his face. 

“It was childish. It was petty,” Schumer told reporters. “It’s something that a five-year-old would do, not a president of the United States. But it shows how unserious they are. They don’t give a damn about the harm they will cause with their shutdown.”

–Reuters