WASHINGTON — US President Donald Trump warned on Thursday that a government shutdown was looming, as the White House raised the stakes in a clash with Democrats by threatening mass federal firings.

Republican Trump is in a tense showdown with congressional Democrats to agree on spending plans ahead of a fiscal deadline of midnight on Sept 30, after which key services will be cut.

The White House earlier ordered government agencies to prepare for layoffs that would go beyond the usual practice of temporary furloughs during government shutdowns.

A shutdown would result in nonessential operations coming to a halt and hundreds of thousands of civil servants being temporarily left without pay.

In a late Wednesday memo, the White House's Office of Management and Budget, or OMB, asked agencies to identify programs, projects and activities where discretionary funding will lapse on Oct 1 if the US Congress does not pass legislation to keep the federal government open.

"Programs that did not benefit from an infusion of mandatory appropriations will bear the brunt of a shutdown," the OMB said in the memo.

The memo told agencies to submit their proposed reduction-in-force plans to the OMB and to issue notices to employees if they are ultimately shut down.

The move would add to the pain of government workers after large-scale firings masterminded by Elon Musk's Department of Government Efficiency earlier this year.

Democrats have rejected Republican proposals unless some of the spending cuts are reversed and existing healthcare subsidies are extended.

"We will not be intimidated," Democratic House Minority Leader Hakeem Jeffries told reporters, describing OMB Chief Russell Vought as "totally and completely out of control".

Shutdown battles have become a regular feature of US politics under both Republican and Democratic administrations, as Washington is increasingly paralyzed by polarization.

The federal government is on the brink of its 15th partial shutdown since 1981, as lawmakers have failed to agree on a plan for discretionary funding for the new fiscal year, which accounts for about one-quarter of the $7 trillion budget.

Congress last faced a shutdown in March, when Republicans refused to talk with Democrats over Trump's massive budget cuts and the layoffs of thousands of federal employees.

'Negotiating tactic'

The shutdown was also a negotiating tactic, with six references in a short document to how Democrats voted against the Republican stopgap funding bill last week, which was hurriedly passed by the House of Representatives as it sought to avert a shutdown.

Trump canceled a meeting on Tuesday with Democratic leaders in Congress, saying he would not meet with them until they "become realistic" with their demands.

With both chambers on recess this week and senators returning on Monday, time is running out to keep the US government funded after the end of the fiscal year.

House Republicans warned on Friday that their members will not return before the funding deadline, forcing the Senate to vote again and accept their proposal or face a shutdown.

The bill, if passed, would still only be a temporary fix, funding federal agencies through Nov 21.

Agencies Via Xinhua