The US Supreme Court has struck down a Louisiana law restricting access to abortion in a key victory for abortion rights activists.

The conservative-leaning court split 5-4 in the decision overruling a state law that requires doctors who perform abortions to have admitting privileges at a nearby hospital.

The Louisiana law "would drastically reduce the number and geographic distribution of abortion providers, making it impossible for many women to obtain a safe, legal abortion in the state," the court's ruling said.

Right-leaning Chief Justice John Roberts sided with the court's four progressives, frustrating abortion opponents who had hoped that President Donald Trump's appointment of two conservative justices had tilted the high court firmly in their direction.

But the judge said his decision to join with the progressives was strictly on institutional grounds, and not because he believed the Louisiana law unconstitutionally restricted access to abortion services.

Instead, Chief Justice Roberts said that he was not prepared to reverse the precedent of the court's 2016 decision - made before President Trump's justices arrived - rejecting an identical Texas law.

In a view that could encourage conservatives to continue to fight for a clear Supreme Court decision against abortion, the judge said he continues to believe that the Texas case was wrongly decided.

However, he said: "The legal doctrine of stare decisis requires us, absent special circumstances, to treat like cases alike."

Nancy Northup, president of the Center for Reproductive Rights, said they were relieved that the court blocked the Louisiana law.

"With this win, the clinics in Louisiana can stay open to serve the one million women of reproductive age in the state," she said in a statement.

"But we're concerned about tomorrow," she said.

"Unfortunately, the court's ruling today will not stop those hell-bent on banning abortion."