US President Donald Trump has suffered a major setback over one of his key campaign pledges, which was to repeal Obamacare and overhaul the country's health insurance system.

A third senator from Trump's own Republican Party said on Tuesday she could not back any legislation to repeal Obamacare without a replacement plan, effectively blocking efforts to bring up a narrower repeal-only bill after earlier legislation collapsed.

"To repeal there has to be a replacement. There's enough chaos already, and this would just contribute to it," Senator Lisa Murkowski of Alaska said.

She said she would vote "no" on any effort to take up legislation to repeal Obamacare, or the Affordable Care Act, enacted in 2010 on then President Barack Obama's watch.

In a senate divided 52-48 between Republicans and Democrats, Senate Majority Leader Mitch McConnell could lose only two senators and still prevail on a procedural vote to open debate on the bill.

On Monday, two other Republican senators announced their opposition to the legislation backed by Trump. 

All opposition Democrats and two leftwing independents had already pledged to fight the changes.

On Tuesday morning, Trump berated Democrats for blocking his plan.

"We were let down by all of the Democrats and a few Republicans," Trump posted on social media. "Most Republicans were loyal, terrific & worked really hard. We will return!"

Source: Al Jazeera and news agencies

US & Canada, United States, Health