US President Donald Trump's administration on Wednesday froze US$26 billion for Democratic-leaning states, following through on a threat to use the government shutdown to target Democratic priorities.

The targeted programs included US$18 billion for transit projects in New York, home to Congress's top two Democrats, and US$8 billion for green-energy projects in 16 Democratic-run states, including California and Illinois.

Vice President JD Vance, meanwhile, warned that the administration might extend its purge of federal workers if the shutdown lasts more than a few days.

The moves made clear that Trump would carry out his threat to take advantage of the shutdown to punish his political opponents and extend his control over the US$7 trillion federal budget, established by the US Constitution as the domain of Congress.

The pressure tactics came as the 15th government shutdown since 1981 suspended scientific research, financial oversight, environmental cleanup efforts and a wide range of other activities.

Some 750,000 federal workers were ordered not to work, while others, such as troops and Border Patrol agents, began to work without pay.

The Department of Veterans Affairs said it would provide burials at national cemeteries, but would not erect headstones or mow the grass.

Vance said at a White House briefing that the administration would be forced to resort to layoffs if the shutdown lasts more than a few days, adding to the 300,000 who will be pushed out by December.

Previous shutdowns have not resulted in permanent layoffs.

The US Patent and Trademark Office said it would lay off 1 percent of its 14,000 employees, according to an internal letter seen by Reuters.

Hakeem Jeffries, the top Democrat in the House of Representatives, said the funding freeze for subway and harbour projects in his home of New York would throw thousands out of work.

Top Senate Democrat Chuck Schumer, also from New York, said Trump was targeting regular Americans for partisan aims.

"He is using the American people as pawns, threatening pain on the country as blackmail," Schumer said.

Republican Senator Thom Tillis said he was concerned that the freezing of infrastructure funds for New York could make it harder for Congress to exit the shutdown.

"They need to be really careful with that, because they can create a toxic environment here," Tillis said.

"So hopefully they're working with the leader, and the leader with them, on not creating more work to get us out of this posture."

Republican Senate Leader John Thune dismissed concerns that the spending freeze amounted to hostage-taking.

"Well, vote to open up the government and that issue goes away, right? I mean, it's pretty straightforward," he said at a press conference.

Meanwhile, the Senate again rejected efforts to keep the government functioning as both a Republican proposal that would fund the government through November 21 and a Democratic vote that would pair funding with additional health benefits failed in floor votes.

Trump's Republicans hold a 53-47 Senate majority, but they need the support of at least seven of Schumer's Democrats to meet the chamber's 60-vote threshold for spending bills.

At issue on the government funding front is US$1.7 trillion for agency operations, which amounts to roughly one-quarter of annual spending.

Much of the remainder goes to health and retirement programs and interest payments on the growing US$37.5 trillion debt. (Reuters)