The US government has begun to inform workers of an impending shutdown that could see millions of federal employees and military personnel sent home or working without pay, unless Congress reaches a last-ditch deal.

Without an agreement, funding for much of the federal government will expire at midnight on Saturday, threatening disruptions to everything from air travel to some benefit payments, and -– if the shutdown endures –- dealing a further blow to the precarious US economy.

The stand-off has been triggered by a small group of hardline Republicans who have pushed back against short-term funding deals, with Congress in deadlock over calls for deep spending cuts.

Some federal employees have been informed of preparations for a lapse, according to a notice seen by AFP.

A note to staff at the Department of Health and Human Services outlined how it would see "reduced staffing across nearly every division for the duration of the lapse" although many key programs will continue.

The department also updated its contingency plans, adding that "pre-notified employees would be temporarily furloughed," meaning they are not allowed to work. They would receive retroactive pay after the lapse ends, the note said.

Staff at other agencies were understood to have received similar notices.

In a shutdown, hundreds of thousands of federal workers would be furloughed without pay, and members of the military and other employees who are deemed to be essential would continue working without a paycheck.

Certain benefits like Social Security checks would not be hit, but workers who go unpaid could eventually stop showing up, impacting sectors like air travel.

- 'Dangerous' -

Apart from the possible lapse in funding, the Federal Aviation Administration (FAA) faces the added headache of a Saturday deadline for reauthorization.

It remains unclear if lawmakers will pass an FAA reauthorization law separately from a spending package.

Transportation Secretary Pete Buttigieg said in a press conference on Wednesday that "there is no good time for a government shutdown," but adding that "this is a particularly bad time."

"The consequences would be disruptive and dangerous," he said.

In Washington, a group of young climate activists of the youth-led Sunrise Movement entered Republican House Speaker Kevin McCarthy's office to demand that a shutdown be avoided.

And the White House warned in a statement that a lapse would leave the Federal Emergency Management Agency's disaster relief fund underfunded, "delaying nearly 2,000 long-term recovery projects" across the country.

- 'Avoidable risk' -

With just days left to pass legislation that would keep the government running, Senate Majority Leader Chuck Schumer, a Democrat, said Thursday that his chamber is "pursuing bipartisanship."

He accused House Speaker McCarthy of choosing to "elevate the whims and desires of a handful of hard-right extremists," with "nothing to show for it."

Mitch McConnell, the Republican Senate minority leader, said shutting down the government is an "actively harmful proposition."

In a full shutdown, the American Federation of Government Employees (AFGE) union estimates almost 1.8 million federal workers would go unpaid for the duration.

A spokeswoman for the International Monetary Fund added in a briefing on Thursday: "We do see a shutdown as an avoidable risk for the US economy."

"We encourage the parties to come together to reach consensus on ways to fund the US government," she said.

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