President Nayib Bukele of El Salvador offered on Monday to jail convicted criminals deported by the United States, a move that won praise from Secretary of State Marco Rubio despite questions about whether it is legal or even possible.

“We have offered the United States of America the opportunity to outsource part of its prison system,” Mr. Bukele wrote on X, saying his government was willing to take in convicted criminals, including U.S. citizens, for a fee. “The fee would be relatively low for the U.S. but significant for us, making our entire prison system sustainable.”

El Salvador was the second stop on Mr. Rubio’s first foreign trip as secretary of state. After meeting with Mr. Bukele on Monday, Mr. Rubio said that he had briefed President Trump on the offer, which he described as unprecedented.

Mr. Rubio said that El Salvador had proposed jailing undocumented migrants who have been convicted of crimes and deported from the United States. The secretary said Mr. Bukele had offered to also accept convicted criminals who are currently serving their sentences in the United States, “even if they are U.S. citizens or legal residents.”

The State Department later added that Mr. Bukele had agreed to take undocumented migrants from any country, not just El Salvador, that have been convicted of crimes, including members of the MS-13 and Tren de Aragua gangs.

Despite the sweeping scope of the offer, which Mr. Rubio described as “an act of extraordinary friendship,” deporting U.S. citizens would fly in the face of protections that make it illegal in all but the rarest of cases.

While details of the plan are not yet known, it is another example of how the Trump administration is quickly driving home the point to governments in the region that they are either allies or enemies based on their willingness to support him, especially on illegal migration, fentanyl trafficking and restricting Chinese influence.

Mr. Bukele rose to power in 2019 on a promise to rid his country of drugs and gangs and has since earned adulation across Latin America for bringing down crime in his country. At the same time, he has used emergency powers to order mass arrests that critics say have trampled human rights and the rule of law, ensnaring thousands of innocent people.

Mr. Bukele said Monday that criminals deported by the United States would go to the Terrorism Confinement Center, a prison built to house 40,000 people. Human rights groups have documented extreme overcrowding in El Salvador’s prisons and reports of torture by guards.

El Salvador signed a similar agreement in 2019 to receive non-Salvadorans detained in the United States, known as a “safe third country” agreement. That deal was never implemented because of the Covid pandemic. Mr. Bukele referred to it on Monday, saying his new proposal was “more important and of a much broader scope than the agreements made in 2019.”

Michael Crowley and Gabriel Labrador contributed reporting from San Salvador, El Salvador.