The US Supreme Court on Thursday rejected a conservative legal challenge that could have doomed President Barack Obama's healthcare law, upholding nationwide tax subsidies crucial to his signature domestic policy achievement.

Obama strode into the White House Rose Garden after the ruling to declare that the law known as Obamacare is working, helping millions of Americans afford health insurance who otherwise would have none, and that it is "here to stay".

Chief Justice John Roberts, a conservative appointed by Republican President George W. Bush, wrote in the 6-3 ruling that Congress clearly intended for the tax subsidies that help millions of low- and moderate-income people afford private health insurance to be available in all 50 states.

The court decided that the law did not restrict the subsidies to states that establish their own online health insurance exchanges, as the challengers in the case contended.

"Congress passed the Affordable Care Act to improve health insurance markets, not to destroy them," Roberts wrote, adding nationwide availability of the credits is required to "avoid the type of calamitous result that Congress plainly meant to avoid."

Roberts was joined by fellow conservative Justice Anthony Kennedy and the court's four liberal members in a ruling that may ensure Obamacare becomes a lasting element of the nation's social programs.

The ruling means the current system will remain in place, with subsidies available nationwide. If the challengers had won, at least 6.4 million people in at least 34 states would have lost subsidies worth an average of $272 per month.

It marked the second time in three years the high court ruled against a major challenge to the law brought by conservatives. Both rulings were written by Roberts. Unlike the 2012 case, in which the court was split 5-4, Kennedy joined Roberts in the majority this time.

The law was passed by Obama's fellow Democrats in Congress in 2010 over the unified opposition of Republicans, who have fought it since its inception.

Republicans will keep attacking Obamacare in Congress and on the 2016 presidential election campaign trail to energize right-wing voters and raise money, but little chance exists of the law being rolled back before the end of Obama's presidency in January 2017, political analysts said.

Obama said the law has been "woven into the fabric of America."

"After more than 50 votes in Congress to repeal or weaken this law, after a presidential election based in part on preserving or repealing this law, after multiple challenges to this law before the Supreme Court, the Affordable Care Act is here to stay," Obama added.