President Donald Trump announced Thursday he intends to sign an executive order "sometime next week" to bar immigrants from seeking asylum in the United States if they crossed the border illegally.

US law currently requires the government to allow immigrants to make asylum requests no matter how they crossed the border.

"Under this plan, the illegal aliens will no longer get a free pass into this country," he said. "Instead, migrants seeking asylum will have to present themselves lawfully at a port of entry."

In a rambling address from the White House's Roosevelt Room on Thursday, President Donald Trump raged against the multiple caravans of thousands of Central American migrants currently en route to the US through the Northern Triangle countries and Mexico.

Trump announced he intends to dramatically limit the ways immigrants can seek asylum in the United States, and will seek to curb their ability to receive asylum if they request it after entering the country illegally, rather than at a legal port of entry.

Though it's unclear whether the caravan migrants intended to seek asylum at ports of entry or cross illegally, Trump painted them as invaders seeking to cross the US-Mexico border illegally.

"At this moment, large, well-organized caravans are marching toward our southern border. Some people call it an 'invasion.' It's like an invasion," he said. "These illegal caravans will not be allowed into the United States and they should turn back now. They're wasting their time."

On his own Twitter account on Monday, Trump had called the caravan an "invasion."

Read more: Thousands of migrant children are trekking hundreds of miles in a caravan to the US. These powerful pictures show their difficult journey.

He said his administration is finalizing an executive order for "sometime next week" about the asylum changes, though he didn't offer details.

The Immigration and Nationality Act currently requires the US government to allow immigrants to make asylum requests no matter how they crossed the border, and it's unclear what actions Trump could take to change that.

"Under this plan, the illegal aliens will no longer get a free pass into this country," Trump said. "Instead, migrants seeking asylum will have to present themselves lawfully at a port of entry."

Critics immediately seized on the vagueness of Trump's announcement, accusing him of stoking anti-immigrant sentiments ahead of the midterm elections next week and lying about the asylum system.

"If he plans at some point to prohibit people from applying for asylum between the ports of entry, that plan is illegal," Omar Jadwat of the American Civil Liberties Union's Immigrants' Rights Project said in a statement. "What is clear from the timing and vague nature of today's remarks is that he is simply trying to inflame his base in the final run-up to the midterms."

Migrants have long sought to claim asylum after illegally crossing the border, and many have even been prevented by US officials from accessing the ports of entry to request asylum.

Last spring, at the height of the "zero tolerance" policy that resulted in family separations, the Trump administration implemented a practice called "metering," in which immigration officers forced migrants to wait for days or weeks outside the ports of entry before being permitted to make an asylum request.

"Those who choose to break our laws and enter illegally will no longer be able to use meritless claims to gain automatic admission into our country," Trump said Thursday. "We will hold them for a long time if necessary."