LOS ANGELES — A federal judge in California on Wednesday issued a preliminary injunction blocking the Trump administration's new rule barring most immigrants from obtaining asylum in the US if they transit through Mexico.

US District Judge Jon Tigar handed down his decision, which takes effect immediately, following a hearing on the controversial policy that the government argued was necessary to stem the flow of migrants into the US.

The policy would have effectively prevented most asylum seekers from Central America from gaining entry into the United States at the southern border as the majority come through Mexico.

Tigar's decision came as another federal judge in Washington heard a separate challenge on Wednesday and allowed the new rule to be implemented.

However, Tigar's ruling means that the new policy cannot be implemented until the legal challenges are resolved.

"This new rule is likely invalid because it is inconsistent with the existing asylum laws," Tigar wrote in his order, denouncing the government's decision to promulgate the new law as "arbitrary and capricious."

The American Civil Liberties Union, the Southern Poverty Law Centre and the Center for Constitutional Rights had challenged the policy in court last week.

Two other advocacy groups had filed a separate challenge in Washington.

"Today's ruling is an important victory for incredibly vulnerable individuals and families from besieged Central American countries seeking refuge in our country," Melissa Crow, an attorney with the Southern Poverty Law Center's Immigrant Justice Project, said in a statement following Tigar's ruling.

"We will continue to fight this draconian policy as well as the myriad of others through which the Trump administration continues to wage war on asylum-seekers and our nation's asylum system."

The lawsuit filed in California argues that the new rule was in violation of immigration laws that clearly state that asylum could not be denied based on a person's route to the United States.

The White House measure targets the stream of hundreds of thousands of migrants from Central America and other countries who have tried to cross into the United States from Mexico and request asylum over the past few months.

These requests -- increasingly made by families saying they have fled endemic violence and poverty in their countries -- allow the applicants to remain in the United States and to move around freely while their cases are adjudicated, which can take two years.

The number of border-crossers detained by the US Border Patrol surged to a 13-year high of more than 144,000 in May before easing to 104,000 in June -- still up 142 percent from a year earlier.

Most are families from Guatemala, Honduras and El Salvador. — AFP