US President Donald Trump said on Monday he wants to deport some violent criminals who are US citizens to Salvadoran prisons, a move that experts said would violate US law.

Trump’s comments marked the clearest signal yet that the US president is serious about deporting naturalised and US-born citizens, a proposal that has alarmed civil rights advocates and is viewed by many legal scholars as unconstitutional.

Trump said he would only go through with the idea if his administration determined it was legal.

It was not clear what level of due process an American would receive before being deported to a country Washington has previously accused of serious human rights abuses, including harsh and arbitrary detentions.

“We always have to obey the laws, but we also have homegrown criminals that push people into subways, that hit elderly ladies on the back of the head with a baseball bat when they’re not looking, that are absolute monsters,” Trump told reporters during Salvadoran President Nayib Bukele’s visit to the White House.

“I’d like to include them in the group of people to get them out of the country, but you’ll have to be looking at the laws on that,” Trump added.

The US government cannot forcibly remove citizens from the country for any reason, though in rare cases, foreign-born citizens can be stripped of citizenship and deported if they commit terrorism or treason or are found to have lied about their background during the naturalization process.

“There is no provision under US law that would allow the government to kick citizens out of the country,” said University of Notre Dame professor Erin Corcoran, an immigration law expert.

Trump told reporters last week that he “loved” the idea of deporting citizens to El Salvador, after Bukele said the country was open to housing US prisoners.

White House press secretary Karoline Leavitt later confirmed that the proposal was on the table, saying Trump had “simply floated” the idea.

The Trump administration has sent hundreds of migrants accused of criminal affiliations to El Salvador’s harsh mega-prison known as the Terrorism Confinement Centre, under often-contested legal authorities. The US is paying El Salvador $6 million to detain the migrants.