US Secretary of State Marco Rubio said Monday that El Salvador's President Nayib Bukele made the extraordinary offer to take in prisoners with US citizenship sent from the United States.

There is virtually no precedent in contemporary times for a democratic country to send its own citizens to foreign jails.

But Rubio said Bukele, who has launched a sweeping crackdown on crime that has won him popularity at home and hero status in President Donald Trump's circles, offered to do just that in their meeting earlier in the day.

"He has offered to house in his jails dangerous American criminals in custody in our country, including those with US citizenship and legal residency," Rubio told reporters.

"No country's ever made an offer of friendship such as this," Rubio said.

"We are profoundly grateful. I spoke to President Trump about this earlier today," he said.

Rubio appeared to suggest that the prison transfer would focus on members of Latin American gangs such as El Salvador's MS-13 and Venezuela's Tren de Aragua who have acquired US citizenship.

"Any unlawful immigrant and illegal immigrant United States who is a dangerous criminal -- MS-13, Tren de Aragua, whatever it may be -- he has offered his jails," Rubio said.

Trump has sought to crack down on the right to birthright citizenship, which is enshrined in the US Constitution.