The US Supreme Court on Wednesday granted a request by President Donald Trump's administration to fully enforce a new rule that would curtail asylum applications by immigrants at the US-Mexico border, a key element of his hard-line immigration policies.

The court said the rule, which requires most immigrants who want asylum to first seek safe haven in a third country through which they had travelled on their way to the United States, could go into effect as litigation challenging its legality continues.

"BIG United States Supreme Court WIN," Trump tweeted after the ruling.

Liberal Justices Sonia Sotomayor and Ruth Bader Ginsburg dissented.

"Once again, the Executive Branch has issued a rule that seeks to upend long-standing practices regarding refugees who seek shelter from persecution," Sotomayor wrote.

The rule, unveiled on July 15, requires most immigrants who want US asylum to first seek asylum in a third country they had travelled through on their way to the US.

The San Francisco-based 9th US Circuit Court of Appeals on August 16 limited a federal judge's injunction blocking the rule to the nine Western states over which it has jurisdiction including the border states of California and Arizona. That had left open the possibility that the rule could be applied in the two other border states, Texas and New Mexico.

The American Civil Liberties Union (ACLU) and others who challenged the administration's policy in federal court said it violates US immigration law and accused the administration of failing to follow the correct legal process in issuing the rule.

"This is just a temporary step, and we're hopeful we'll prevail at the end of the day. The lives of thousands of families are at stake," said Lee Gelernt, the American Civil Liberties Union lawyer who is representing immigrant advocacy groups in the case.

The rule would block nearly all families and individuals from countries like El Salvador, Honduras and Guatemala from entering the US after crossing through Mexico as asylum seekers. The rule has some exceptions and would keep asylum protections for Mexican citizens. But it would apply to the majority of asylum seekers from Africa, Asia and elsewhere in South America.

Many Central Americans who have travelled to the US border over the last year have told Al Jazeera they were fleeing violence, poverty and political persecution.

The shift reverses decades of US policy. US law allows refugees to request asylum when they arrive in the US regardless of how they did so, but there is an exception for those who have come through a country considered to be "safe". The Immigration and Nationality Act, which governs asylum law, is vague on how a country is determined "safe". The act says that a safe country is "pursuant to a bilateral or multilateral agreement".

Right now, the US has such an agreement, known as a "safe third country", only with Canada. The US and Guatemala have also signed a safe country agreement, but the details are unclear and Guatemalan President-elect Alejandro Giammattei has said the deal still needs congressional approval. (Al Jazeera)

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