The Trump administration is assessing whether it could take up El Salvador's offer to house U.S. citizens convicted of crimes in its jails, Secretary of State Marco Rubio said on Tuesday, despite the apparent illegality of such a move.

El Salvador's President Nayib Bukele on Monday offered the U.S. "the opportunity to outsource part of its prison system," he said on X after meeting Rubio.

Rubio, asked about the proposal during a news conference in Costa Rica on Tuesday, called it "an incredible offer, an unprecedented one."

"Obviously, we'll have to study it on our end; there are obviously legalities involved. We have a constitution, we have all sorts of things, but it's a very generous offer," he said.

The U.S. Constitution provides protections for citizens. Courts have ruled that Americans cannot be stripped of their citizenship for committing crimes, meaning they cannot be deported.

President Donald Trump later said he would enact the plan "in a heartbeat" if he had the legal right to do so, and said it would be cheaper than holding inmates in private U.S. prisons.

"We're looking at that right now, but we could make deals where we'd get these animals out of our country," Trump told reporters at the White House.

Trump himself suggested last month that the U.S. could pay foreign countries "a very small fee" to imprison Americans who are repeat criminal offenders, billing the idea as a cost-saving measure.

As well as the legal question, the plan would also likely face opposition due to the conditions in El Salvador's jails.

The U.S. State Department's website says prison conditions in El Salvador are "harsh and dangerous" and notes risks from overcrowding.

Bukele has launched an unflinching security crackdown in his country, arresting more than 80,000 people, and bringing the number of homicides down sharply.

He offered the U.S. use of his Terrorism Confinement Center mega-prison for both foreign and U.S. criminals for a fee.

Since taking office on Jan. 20, the Trump administration has stepped up the number of migrants the U.S. deports to Latin America, including using military planes for repatriation flights.

On his X platform on Monday night, Elon Musk, the billionaire heading Trump's drive to shrink the federal government and cut costs, called Bukele's proposal a "Great idea!!"