Republicans on Saturday mapped out a collision course towards the first US government shutdown in 17 years, making fresh demands for Barack Obama's heathcare reforms to be postponed that are almost certain to be rejected by Democrats.

With barely 48 hours to go until existing federal government spending authority expires on Monday night, House Republicans agreed to pass a continuing budget resolution until December, but only if Obamacare is delayed for a year and stripped of a key tax on medical devices.

But Senate majority leader Harry Reid issued a statement on Saturday saying his chamber will reject the House Republicans' plan. He said House passage of any attempt to delay the healthcare law woud be pointless.

House Republicans also plan to pass separate legislation ensuring that US troops continue to receive pay during any ensuing shutdown, exempting a politically sensitive area of federal government from the consequences of their high-stakes clash with Obama.

The main spending bill will be put before the full House of Representatives later on Saturday, but with the majority Republican caucus seemingly united in its desire for a showdown over Obamacare, it is now all but certain that the spending resolution will be passed back a second time to the Senate.

The Senate has already rejected one House attempt to link spending authorisation to Obamacare and its Democrat majority leader Harry Reid has pledged to block anything but a "clean" bill.

Obama has accused Republicans of holding the US economy to ransom and has upped his rhetoric in recent days to make it clear he would also veto any resolution that involved Obamacare.

House speaker John Boehner refused to speak to reporters after his meeting with Republicans on Saturday afternoon, although he is expected to begin outlining the plan on the floor of the House.

The last time the US government was deprived of funding in this way was under Bill Clinton in 1995 and 1996, when he clashed with Republican speaker Newt Gringrich.

Under the Anti-Deficiency Act, passed after the American civil war, the federal government is forbidden from incurring costs that have not been explicitly authorised by Congress.

Only staff involved in "emergencies involving the safety of human life or the protection of property" are exempt which in practice means many "essential workers" deemed vital to security and law enforcement.

But hundreds of thousands of other federal employes will be "furloughed" or told to stay at home from Tuesday morning if Congress cannot find a way around the growing impasse. Social security and other benefit payments may also be delayed.

In a speech on Friday, Obama warned that active-duty military employees could see their pay disrupted but the Republican plan to exempt US armed forces removes one area of leverage that might have forced conservatives to back down.