Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside the Supreme Court in Washington DC, the US on June 27, 2016. Photo: AFP

The US Supreme Court on Monday struck down a Louisiana law that tightly restricted access to abortion in the first constitutional test of abortion rights since President Donald Trump named two conservative justices to the top court.

In a key victory for abortion rights activists, the justices voted 5-4 to overrule 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 ruling said.

Right-leaning Chief Justice John Roberts sided with the court's four progressives, frustrating abortion opponents who had hoped that Trump's appointments had tilted the high court firmly in their direction.

But Roberts gave them hope that a different challenge to abortion rights might succeed in the future in the conservative court.

He said his decision to join with the progressives in the case, June Medical Services v Russo, was strictly on institutional grounds, and not because he believed the Louisiana law was unconstitutional.

Instead, Roberts said that he was not prepared to reverse the precedent set by the court's 2016 decision rejecting an identical Texas law.

Roberts said that he continues to believe that the Texas case was wrongly decided.

The White House sharply criticized the decision, saying "unelected justices" intruded on the powers individual states have to set policies.

"In an unfortunate ruling today, the Supreme Court devalued both the health of mothers and the lives of unborn children," said a statement from spokeswoman Kayleigh McEnany.

"States have legitimate interests in regulating any medical procedure - including abortions - to protect patient safety," McEnany said.

Supporters of the Louisiana and previous Texas laws had argued that clinics providing abortion services had substandard health and safety practices, an argument pro-abortion groups said was false.

Both sides saw the Louisiana case as a test of the court's view of abortion as a constitutional right.

Abortion foes have in their sights the landmark 1973 Roe v Wade case, in which the Supreme Court legalized a woman's right to an abortion.

AFP