A United States (US) federal judge temporarily blocked Donald Trump's administration from denying asylum to people who enter the country illegally, prompting the president to allege on Tuesday that the court was biased against him.

Trump issued a proclamation earlier this month saying that only people who enter the US at official checkpoints — as opposed to sneaking across the border — can apply for asylum, as a caravan of thousands of Central American migrants made its way north through Mexico.

But US District Judge Jon Tigar in San Francisco issued a temporary restraining order against the Trump proclamation, thus granting a request from human rights groups that had sued shortly after the order was announced.

“It's a disgrace,” Trump said, alleging judicial bias and appearing to refer to Tigar — who was appointed by former president Barack Obama — as an “Obama judge".

“We will win that case in the Supreme Court of the United States,” Trump said.

Tigar wrote that the Immigration and Nationality Act of 1965 (INA) states that any foreigner who arrives in the US, “whether or not at a designated port of arrival”, may apply for asylum.

"The rule barring asylum for immigrants who enter the country outside a port of entry irreconcilably conflicts with the INA and the expressed intent of Congress," Tigar wrote.

“Whatever the scope of the president's authority, he may not rewrite the immigration laws to impose a condition that Congress has expressly forbidden.”

The judge's restraining order remains in effect until the court decides on the case.

Trump's administration has argued that he has the executive power to curb immigration in the name of national security; a power he invoked right after taking office last year with a controversial ban on travellers from several mostly Muslim countries.

'Broken' asylum system

The final version of the order was upheld by the US Supreme Court on June 26 after a protracted legal battle.

“Our asylum system is broken, and it is being abused by tens of thousands of meritless claims every year,” a joint statement from the Justice Department and the Department of Homeland Security said after the asylum ruling.

“We look forward to continuing to defend the executive branch's legitimate and well-reasoned exercise of its authority to address the crisis at our southern border.”

When the new policy was announced by the Department of Homeland Security on Nov 8, a senior administration official said it would address what he called the “historically unparalleled abuse of our immigration system” along the border with Mexico.

Administration officials say anyone who manages to get across can request asylum and subsequently often vanish while their case languishes in the court system.

“The vast majority of these applications eventually turn out to be non-meritorious,” a senior administration official said, asking not to be identified.

Less than 10 per cent of cases result in asylum being granted, the government says.

Human rights campaigners and other critics of the Trump crackdown say that by restricting asylum seekers to border crossing points — which are already under enormous pressure — the government is effectively shutting the door on people who may truly be fleeing for their lives.

"The government cannot abdicate its responsibility toward migrants fleeing harm," the New York Immigration Coalition advocacy group said.

But the administration official argued that “what we're attempting to do is trying to funnel credible fear claims, or asylum claims, through the ports of entry where we are better resourced". That way, the official said, courts will “handle those claims in an expeditious and efficient manner, so that those who do actually require an asylum protection get those protections".

In 2018, border patrols registered more than 400,000 illegal border crossers, according to the Department of Homeland Security. And in the last five years, the number of those requesting asylum has increased by 2,000 per cent, it said.

'Criminals' in caravan

Speaking at the border in Imperial Beach, California, Homeland Security chief Kirstjen Nielsen doubled down, threatening to arrest or deport all caravan migrants who dare to cross into the US.

“The crisis is real and it is just on the other side of this wall,” she said. “Make no mistake — we are very serious. You will not get into our country illegally... If you try to enter our country without authorisation, you have broken the law of the United States, you will be detained, prosecuted and repatriated.”

She claimed that at least 500 of the caravan migrants have been identified as "criminals", without offering up proof or further details on the nature or origin of the claim.