Washington DC - President Donald Trump's White House told federal agencies to prepare for more mass firings during a possible government shutdown next week, according to a memo first published by Politico.

The memo by the Office of Management and Budget indicated that the Trump administration would go beyond the usual practice of temporary furloughs during previous shutdowns.

Sent to US media on Wednesday, the note instructed federal agencies to target programs they are not legally required to continue.

"Programs that did not benefit from an infusion of mandatory appropriations will bear the brunt of a shutdown," the memo said, ordering agencies to submit their proposed staff reduction plans and notify employees.

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It comes as the Trump administration clashes with congressional Democrats over federal funding ahead of a fiscal deadline of midnight on September 30.

Democrats in the Senate rejected a stopgap funding bill last week hurriedly passed by the Republican-controlled House of Representatives seeking to avert a shutdown.

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

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

A shutdown would see non-essential operations grind to a halt and hundreds of thousands of civil servants temporarily left without pay.

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.

However, the bill, if passed, would still only be a temporary fix funding federal agencies through November 21.