El Salvador's leader has made the offer to jail Americans sent by their own country, US Secretary of State Marco Rubio said on Monday.

There is virtually no precedent in contemporary times for a democratic country to send its own citizens to foreign jails, and any attempt to do so is sure to be challenged in US courts.

But Rubio welcomed an offer to do just that by El Salvador's President Nayib Bukele, whose sweeping crackdown on crime has won him soaring popularity at home and hero status for many in US President Donald Trump's orbit.

"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 in San Salvador.

"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 said that Bukele was also willing to take back Salvadoran citizens as well as nationals of other countries.

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 in the United States who is a dangerous criminal -- MS-13, Tren de Aragua, whatever it may be -- he has offered his jails," Rubio said.

Since his return to the White House last month, Trump has put a top priority on speeding up the deportation of millions of people in the United States without legal status.

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

Trump has also unveiled plans to detain 30,000 migrants at the US base in Guantanamo Bay, Cuba -- a prison which previous Democratic presidents hoped to close. (AFP)