The US government began Thursday to inform workers of an impending shutdown that could see millions of federal employees and military personnel temporarily 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 (0400 GMT Sunday), threatening disruptions to everything from air travel to 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 while Congress tries to resolve a broader deadlock over calls for deep spending cuts.

Some federal employees have already 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 notifications.

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' 

"If there is a shutdown in just a few days, our service members would be required to continue working but would be doing so without pay, and hundreds and thousands of their civilian colleagues would be furloughed," Deputy Pentagon Press Secretary Sabrina Singh said Thursday.