Washington (AFP) - US Republican efforts to reform health care hit a roadblock Thursday when three senators threatened to oppose a partial repeal of Obamacare unless House leaders pledged to negotiate further -- but that demand later appeared to be met.

The sudden announcement by Senators Lindsey Graham, Ron Johnson and John McCain threw their party's effort to overhaul Barack Obama's health care reforms into turmoil, highlighting the tensions and infighting over the path forward.

The Senate had been due to hold a critical vote on the measure later Thursday, but the fate of that vote was unclear.

The trio stood opposed to the plan that appeared most likely to be considered later Thursday or early Friday: a measure known as a "skinny repeal," which would remove some Obamacare taxes and the requirement that individuals have health insurance, but keep significant provisions of the law intact.

The idea was that the "skinny repeal" would merely be a vehicle to get the Senate and House of Representatives to a so-called "conference" -- where they would then negotiate a more comprehensive bill.

But senators got nervous that the House might turn around and simply pass the Senate draft, thereby sending it to Trump for his signature.

"There's an increasing concern on my part and others that what the House will do is take whatever we pass... vote on it and that goes to the president's desk with the argument this is better than doing nothing," Graham told reporters.

"I'm not going to vote for a bill that is terrible policy and horrible politics just because we have to get something done," added Graham, who called the Senate legislation "a disaster as a policy."

House Speaker Paul Ryan later said his side was willing to negotiate -- but he also challenged the Senate to actually pass some kind of measure.

"The House remains committed to finding a solution and working with our Senate colleagues, but the burden remains on the Senate to demonstrate that it is capable of passing something that keeps our promise," Ryan said.

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Graham's office said he was reviewing Ryan's statement.

- 'Wild West' -

Republicans struggled for a third straight day Thursday with how to forge consensus on health care reform, with Senate leaders increasingly intent on ramming through the partial repeal.

As the 100-member Senate was bracing for a marathon session carrying into the wee hours of Friday without a break, most lawmakers still had not seen the final bill they will be voting on, even though it now stands at the heart of their seven-year promise to overhaul the Affordable Care Act put in place under Obama.

Senators appeared desperate for any news of what Majority Leader Mitch McConnell was crafting.

They have been prodded and cajoled and strong-armed by President Donald Trump, who campaigned relentlessly on ending the Obamacare "nightmare."

"Come on Republican Senators, you can do it on Healthcare," he nudged in a Thursday morning tweet.

"After 7 years, this is your chance to shine! Don't let the American people down!"

But the mood on Capitol Hill was somber. Asked what is in the bill that will impact perhaps tens of millions of Americans, Senator Bob Corker conceded: "That's not firm at this moment."

"We're in sort of the Wild West right now," Corker told AFP.

- 'Skinny' repeal? -

After two stinging defeats in two days, including failure to pass the Senate's most comprehensive repeal-and-replace legislation, Republicans were looking to seize on something -- anything -- that they can drag across the finish line.

The House passed its health care legislation in May. If House lawmakers do not want to take up what the Senate produces, the two sides would need to negotiate a bill that Congressional leaders believe would pass both chambers.

A non-partisan study of the "skinny repeal" forecast that health care premiums would spike, and 16 million more Americans would lose their health insurance as compared to current law.

Many Republicans acknowledged the bill's faults, but they were left in the extraordinary position of likely voting for a measure they do not want to become law.

Republicans controls 52 seats in the Senate, and can only afford three defectors.

Two Republican senators, Lisa Murkowski and Susan Collins, have consistently voted against Obamacare repeal bills, arguing the plans' provisions could end up kicking millions of people off Medicaid, the health insurance program for the poor and the disabled.

If they alone vote against any measure, Vice President Mike Pence can cast a tie-breaking vote. If a third senator joins them, the measure would fail.

McCain called for work to begin on a bipartisan health care plan, warning: "Otherwise, we'll see this continuous gridlock."