U.S. President Donald Trump on Friday threatened to release detained undocumented immigrants into so-called sanctuary cities, while blaming the country's immigration laws on Democrats.

"Due to the fact that Democrats are unwilling to change our very dangerous immigration laws, we are indeed, as reported, giving strong considerations to placing Illegal Immigrants in Sanctuary Cities only," Trump wrote in a tweet.

The president went on accusing the "radical left" of supporting open borders, claiming that his proposal "should make them very happy!"

Sanctuary cities in the United States are known as local jurisdictions that refrain from cooperating with federal authorities and detain undocumented immigrants for possible deportation.

Trump's threat came a day after The Washington Post reported that administration officials had proposed months ago transporting undocumented immigrants to cities that shield them from deportation.

The proposal has drawn sharp criticism from House Speaker Nancy Pelosi, a California Democrat.

"The extent of this administration's cynicism and cruelty cannot be overstated," a spokeswoman for Pelosi said in a statement to The Post.

The White House and the Department of Homeland Security on Thursday denied that the proposal was still under consideration.

Trump has been reshuffling the Department of Homeland Security, which oversees immigration and border security, in push for a tougher direction in immigration enforcement.