US President Donald Trump has demanded a make or break vote on a key healthcare bill in the House of Representatives, threatening to leave "Obamacare" in place and move on to other issues if Friday's vote fails.

The risky move, which was considered part gamble and part threat, was presented to Republican politicians behind closed doors on Thursday night (local time).

It came after a long and intense day that saw a planned vote on the healthcare bill scrapped, as the legislation remained short of votes amid cascading negotiations among conservative politicians, moderates and others.

What is Obamacare? Affordable Care Act passed by Congress in 2010

Affordable Care Act passed by Congress in 2010 Promised to help tens of millions of uninsured Americans get health coverage

Promised to help tens of millions of uninsured Americans get health coverage Under the plan, people can buy cheap insurance on healthcare.gov

Under the plan, people can buy cheap insurance on healthcare.gov Most coverage costs less than $US100 per month

Most coverage costs less than $US100 per month Policies vary according to person's income, location, family size and level of coverage desired

Policies vary according to person's income, location, family size and level of coverage desired More than 10 million people now have medical cover under the laws

More than 10 million people now have medical cover under the laws Number of uninsured adults reduced by 26 per cent

At the end of it, the President had had enough and was ready to vote and move on, whatever the result, Mr Trump's budget director, Mick Mulvaney said.

"'Negotiations are over, we'd like to vote tomorrow and let's get this done for the American people. That was it," Republican Duncan Hunter of California said as he left the meeting, summarising Mr Mulvaney's message.

"Let's vote," White House chief strategist Steve Bannon said, as he walked out.

"For seven-and-a-half years we have been promising the American people that we will repeal and replace this broken law because it's collapsing and it's failing families, and tomorrow we're proceeding," House Speaker Paul Ryan said.

The outcome of the vote was impossible to predict, with both conservative and moderate politicians claiming the bill lacked votes after a long day of talks.

But the White House appeared ready to gamble that the prospect of failing to repeal former President Barack Obama's health law, after seven years of promising to do exactly that, would force politicians into the "yes" column.

"It's done tomorrow or Obamacare stays," said Republican Chris Collins, a top ally of Mr Trump in the House.

Mr Collins was among those predicting success on Friday, but others did not hide their anxiety about the result.

Asked whether Republicans would be unified on the vote, Republican Matt Gaetz of Florida said, "I sure hope so, or we'll have the opportunity to watch a unified Democratic caucus impeach Donald Trump in two years when we lose the majority."

Key changes

The Republican legislation would halt Mr Obama's tax penalties against people who do not buy coverage and cut the federal-state Medicaid program for low earners, which the statute had expanded.

It would provide tax credits to help people pay medical bills, though generally skimpier than Mr Obama's statute provides.

It also would allow insurers to charge older Americans more and repeal tax boosts the law imposed on high-income people and health industry companies.

Furthermore, the measure would block federal payments to Planned Parenthood for a year, another stumbling block for Republican moderates.

AP