The US government's new rules aimed at deterring illegal crossings on the southern border were rejected on Tuesday by a federal judge in California who said the restrictions on asylum seekers violated federal law.

Judge Jon Tigar of the California Northern District Court in Oakland blocked the Biden administration's rules, which make it harder for migrants to get asylum if they crossed the border illegally or if they didn't first ask for humanitarian refuge in Mexico or another country on their journey to the US.

Those found ineligible for asylum under the rule could face swift deportation to their home country or Mexico and a five-year ban from re-entering the US. Those who attempt to re-enter the country illegally could face criminal charges.

Civil rights and immigrant advocacy groups immediately challenged the policy in court after it was implemented in May, saying it "places vulnerable asylum seekers in grave danger and violates US asylum laws".

Tigar wrote in his ruling that "noncitizens who enter between ports of entry, using a manner of entry that Congress expressly intended should not affect access to asylum". He said the rules were "both substantively and procedurally invalid".

The judge put the ruling on hold for 14 days for a possible appeal. The Justice Department has said it would appeal the ruling.

"The Justice Department disagrees with the district court's ruling today," a spokesperson said in a statement, calling the policy a "lawful exercise of the broad authority granted by the immigration laws".

The ruling was applauded by immigrant advocates as "an important victory" for asylum seekers.

"The ruling is a victory, but each day the Biden administration prolongs the fight over its illegal ban, many people fleeing persecution and seeking safe harbor for their families are instead left in grave danger," said Katrina Eiland, deputy director of the American Civil Liberties Union's (ACLU) Immigrants' Rights Project. The organization represents the plaintiffs in the lawsuit.

The advocates accused the Biden administration's rules of mirroring two Trump-era policies, which they called "entry" and "transit" bans. One of those rules disqualified migrants from asylum if they entered the US in between ports of entry, while the other barred migrants from asylum if they failed to seek protection in a third transit country.

The ACLU successfully challenged both rules and persuaded judges to halt them. An appeal court upheld the lower court's rulings in both cases.

While the new restriction is based on penalizing migrants for entering the US without permission and for not seeking asylum elsewhere, the Justice Department argued that it was less restrictive than the earlier version by former president Donald Trump.

The Biden administration's asylum restriction includes exemptions for unaccompanied children and those who can obtain an appointment to present themselves at a border port through a mobile application called CBP One, which migrants can use to schedule interviews.

More than 38,000 migrants were allowed into the US along border ports of entry in June after securing an appointment through the app, according to government data.

But migrant advocates called the app "flawed" because migrants face limited resources to acquire a smartphone or access internet, and other barriers such as language and illiteracy.

Under US asylum law, migrants on the US soil are allowed to request protection, regardless of how they entered the country. However, the legal threshold to win asylum and a chance to become a permanent resident is high.

The number of migrants crossing the border illegally dropped sharply in May and June, which the Biden administration attributed to the new restrictions.

The administration has rolled out a series of measures to try to reduce migrant crossings on the US-Mexico border but is facing multiple lawsuits from Republican states as well as advocates.