US President Donald Trump said on Friday he is seriously considering funneling detained illegal migrants into the self-declared sanctuary cities that oppose his tough immigration policies.

Trump's announcement on Twitter reversed a previous White House assurance that the idea -- criticized as political retribution against cities led by opposition Democrats -- had been dropped.

"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," Trump said.

"The Radical Left always seems to have an Open Borders, Open Arms policy -- so this should make them very happy!" he tweeted.

The Washington Post revealed the proposal, which would mark a new flashpoint in the ever growing political divide over immigration.

Trump has staked his presidency on his insistence that the United States is being overrun by migrants and asylum seekers.

But opponents, mostly in the Democratic Party, say his push for building more walls on the Mexican border and his almost daily denunciations of migrants as dangerous criminals incite racial hatred.

In comments to reporters later, Trump said that he wanted Congress, where the House of Representatives is controlled by Democrats, to tighten visas and streamline the deportation process.

Otherwise, he warned, he'll order migrants to be dumped on the sanctuary cities.

"If they don't agree we might as well do what they say they want.... We'll bring them to sanctuary city areas and let that particular area take care of it," he said. "We can give them an unlimited supply."

And late Friday, he reiterated his call for a "merit" based immigration system.

"If the Radical Left Democrats all of a sudden don't want the Illegal Migrants in their Sanctuary Cities (no more open arms), why should others be expected to take them into their communities?" he tweeted.

Sanctuary cities are places where local authorities usually Democratic-run, have refused to hand over undocumented immigrants for deportation.

San Francisco Mayor London Breed dismissed Trump's proposal.

"This is just another in a long line of scare tactics and half-baked ideas that are just about chasing headlines and distracting people from real issues," she tweeted.

Citing homeland security officials and leaked emails, the Post said White House officials first broached the plan in November, asking several agencies whether members of a caravan of migrants could be arrested at the border and then bussed "to small- and mid-sized sanctuary cities".

The White House told the Immigration and Customs Enforcement agency (ICE) that the plan was intended to alleviate a shortage of detention space, but would also send a message to Democrats, the Post said.

The suggested policy was pushed back within ICE, with a top official saying it was rife with concerns and noting "there are PR risks as well".

After the White House pressed the issue again earlier this year, ICE rebuffed it as "inappropriate."

The Post said a White House official and a spokesman for the Department of Homeland Security said the proposal was no longer under consideration.

"This was just a suggestion that was floated and rejected, which ended any further discussion," a White House statement said.

One of the centers targeted was the California constituency of House Speaker Nancy Pelosi, whose office on Thursday blasted the plan.

"Using human beings, including little children as pawns in their warped game to perpetuate fear and demonize immigrants is despicable," said Pelosi spokeswoman Ashley Etienne.

Trump's all-out fight with the Democrats over immigration comes as border agents report being overwhelmed by the number of migrants and people demanding asylum after fleeing violent Central American countries.

The number of immigrant detainees in ICE custody has approached 50,000 in recent months, the Post said, an all-time high that has strained the agency's budget.

Another of Trump's controversial proposals to resolve the situation has been to shut the border with Mexico temporarily. He backed off the idea after outcry over the likely damaging economic effects.

In a new twist, The New York Times reported Friday that the president last week told Kevin McAleenan, the soon to be named acting secretary of homeland security, that he still wanted the border closed.

According to the Times, Trump told McAleenan that if he had any legal difficulties as a result of this, he would give him a presidential pardon.