A healthcare bill by US Republicans has cleared a key congressional committee, despite concerns that the plan's impact on the budget remains unclear.
The House of Representatives Ways and Means Committee approved the bill by a 23-16 vote along party lines in the pre-dawn hours of Thursday.
Republicans, who control both chambers of Congress, hope to pass their replacement for Obamacare by mid-April.
But it faces resistance from Democrats, hospitals and even some Republicans.
"This is an historic step, an important step in the repeal of Obamacare," said Republican Representative Kevin Brady, chairman of the panel.
It approved the bill, which was unveiled on Monday, after nearly 18 hours of debate.
The committee made no changes to the draft legislation - known as the American Health Care Act - despite Democratic attempts to introduce amendments.
The chamber's Energy and Commerce Committee was still working through its own marathon session as of Thursday morning.
The new plan would dismantle much of Mr Obama's 2010 Affordable Care Act.
- limits future federal funding for Medicaid, which covers low-income people
- abolishes the requirement that everyone should be insured
- replaces subsidies with tax credits
But the American Hospital Association (AHA), which represents about 5000 hospitals and health networks, said current provisions for "our most vulnerable" would be thrown into doubt under the plan.
The president of the AHA said in a letter to Congress the ability to assess the bill was "severely hampered" by the lack of a proper cost estimate.
The Congressional Budget Office is not expected until next week to put a price tag on the proposed overhaul of the more than $US3 trillion US healthcare system.
Democrats continue to argue that it is impossible to push through a bill without knowing its cost and how many Americans would be affected.
Senator Tom Cotton, a conservative Republican from Arkansas, shared that concern in a series of tweets on Thursday after the vote.
He told House members to "pause, start over" and "get it right, don't get it fast".
"What matters in the long run is better, more affordable [healthcare] for Americans, NOT House leaders' arbitrary legislative calendar," he added.
Some moderate Republicans are also concerned people will be stripped of cover by the legislation.
President Donald Trump is meanwhile exercising his vaunted salesman skills to rally support behind the health bill.
On Thursday, he tweeted that "healthcare is coming along great" and "it will end in a beautiful picture!"
House Representative Tom Cole, who joined lawmakers at a meeting with the president on Wednesday, told Bloomberg Mr Trump had suggested weekly meetings.
Some lawmakers have been invited to hang out on Thursday at the White House's bowling alley, according to the report.
Mr Trump has dined with former foes including Senator Lindsey Graham and Senator Ted Cruz, who brought his family to the White House on Wednesday night.
Overall, the plan is expected to cover fewer people than those who gained insurance under the Obama administration's Affordable Care Act.