The US government is partially shutting down, after President Donald Trump failed to get enough support for funding his US-Mexico border wall.

Trump demanded US$5 billion (NZ$7.5b) to start erecting his cherished Mexican border wall, a chaotic postscript for Republicans in the waning days of their two-year reign controlling government.

This is the third time Trump's Government has shut down in 2018.

"It's really up to the Democrats - totally up to the Democrats as to whether we have a shutdown." - President Donald Trump.

US Vice President Mike Pence, Trump son-in-law and senior adviser Jared Kushner and White House budget chief Mick Mulvaney left the Capitol late Friday (Saturday NZ time) after hours of bargaining with congressional leaders produced no apparent compromise. "We don't have a deal. We're still talking," Senate Appropriations Committee Chairman Richard Shelby, told reporters.

Late Friday (local time), Mulvaney sent agency heads a memorandum telling them to "execute plans for an orderly shutdown". He wrote that administration officials were "hopeful that this lapse in appropriations will be of short duration" - an expectation that was widely shared.

Trump's Government has shut down for a third time this year, after the senate refused to fund his border wall.

With negotiations expected to continue, the House and Senate both scheduled rare Saturday (local time) sessions. House members were told they'd get 24 hours' notice before a vote.

The gridlock blocks money for nine of 15 Cabinet-level departments and dozens of agencies, including the departments of Homeland Security, Transportation, Interior, Agriculture, State and Justice.

The lack of funds will disrupt many government operations and the routines of 800,000 federal employees. Roughly 420,000 workers were deemed essential and will work unpaid just days before Christmas, while 380,000 will have leave, meaning they'll stay home without pay.

Those with leave include nearly everyone at Nasa and 52,000 workers at the Internal Revenue Service. About eight in 10 employees of the National Park Service will stay home and many parks were expected to close.

The Senate passed legislation ensuring workers will receive back pay, which the House seemed sure to approve.

Some agencies, including the Pentagon and the departments of Veterans Affairs and Health and Human Services, were already funded for the year in agreements reached earlier, and they will operate as usual.

Trump predicted that the shutdown "will last for a very long time."

The US Postal Service, busy delivering packages for the holiday season, will not be affected because it's an independent agency. Social Security checks will still be mailed, troops will remain on duty and food inspections will continue.

Also still functioning will be the FBI, the Border Patrol and the Coast Guard. Transportation Security Administration officers will continue to staff airport checkpoints and air traffic controllers will also remain at work.

More at https://www.stuff.co.nz/world/americas/donald-trumps-america/109562060/us-government-shutdown-all-but-certain-as-senate-rejects-mexican-wall-funding