Pro-choice activists go emotional after the Senate approved a bill to legalize abortion outside the Congress in Buenos Aires, Argentina on Wednesday. Photo: AFP

The US Supreme Court refused on Wednesday to block a Texas ban on abortion after six weeks of pregnancy, dealing a major blow to abortion rights by leaving in place a state law that prohibits the vast majority of abortions.

The decision is a major milestone in the fight over abortion, as opponents have sought for decades to roll back access to the procedure.

By a 5-4 vote, the justices denied an emergency request by abortion and women's health providers for an injunction on enforcement of the ban, which took effect early on Wednesday, while litigation continues.

One of the court's six conservatives, Chief Justice John Roberts, joined its three liberals in dissent.

"The court's order is stunning," liberal Justice Sonia Sotomayor wrote in a dissenting opinion.

"Presented with an application to enjoin a flagrantly unconstitutional law engineered to prohibit women from exercising their constitutional rights and evade judicial scrutiny, a majority of justices have opted to bury their heads in the sand."

In an unsigned explanation, the court's majority said the decision was "not based on any conclusion about the constitutionality of Texas's law" and allowed legal challenges to proceed.

The decision illustrates the impact of former Republican president Donald Trump's three conservative appointees, who have tilted the court further right. All were in the majority.

The law would amount to a near-total ban on the procedure in Texas, as 85 percent to 90 percent of abortions are obtained after six weeks of pregnancy, and would probably force many clinics to close, abortion rights groups said.

Such a ban has never been permitted in any state since the Supreme Court decided Roe v. Wade, the landmark ruling that legalized abortion nationwide, in 1973. The court's action over the Texas ban could foreshadow its approach in another case over a 15-week ban by Mississippi in which the state has asked the justices to overturn Roe v. Wade.