White House officials have tried to pressure U.S. immigration authorities to release migrants detained at the border into so-called sanctuary cities such as San Francisco to retaliate against President Donald Trump's political adversaries, the Washington Post reported on Thursday.

The Post, which reviewed emails on the issue and spoke to unnamed officials at the Department of Homeland Security (DHS), said the White House proposed the measure at least twice in the past six months. Sanctuary cities are those where local officials decline to hand over illegal immigrants for deportation.

The White House did not immediately respond to a Reuters request for comment on the report. A DHS spokesman told Reuters in a statement the plan was "a suggestion that was floated and rejected, which ended any further discussion." The Post quoted a White House official as saying the same thing.

Trump administration officials proposed the measure in November as a caravan traveled through Mexico with mostly migrants from Central American countries towards the southern U.S. border. 

The proposal emerged again in February during a standoff with Democrats over funding the president sought to build a wall on the border, one of the signature issues of his 2016 election campaign and presidency.

The Post said a Nov. 16 email broached the proposal, asking officials at different agencies whether members of the migrant caravan could be detained at the border, then bused to "small- and mid-sized sanctuary cities," where local officials refuse to hand over illegal immigrants for deportation.

House Speaker Nancy Pelosi's district in San Francisco was one of those the White House considered targeting, the Post cited the DHS officials as saying.

Ashley Etienne, a spokeswoman for Pelosi, denounced the administration for its "cynicism and cruelty" over the plan.

"Using human beings, including little children, as pawns in their warped game to perpetuate fear and demonize immigrants is despicable, and in some cases, criminal," she said, adding that Americans had "resoundingly rejected this administration's toxic anti-immigrant policies."

Deaths of two migrant children in detention last year prompted criticism from internal and international societies. There are many vulnerable children and women currently in the custody of U.S. immigration authorities. 

When they were caught by border patrols, they were supposed to be detained by Customs and Border Protection (CBP) firstly for no more than 72 hours before being turned over to other government agencies like Immigration and Customs Enforcement (ICE)  which is responsible for long-term detention. 

CBP facilities are typically spare in accommodations, offering food, water and blankets, but medical professionals, teachers or some of the other resources that longer-term detention centers offer are challenged to provide.

According to the latest modified version of CBP, the total number of detained people on the southwest border of the U.S. is 361,087, including 35,898 unaccompanied alien children.

The Guardian reported in December last year that  although immigration officials have been arrested for the sexual abuse of children in their care, the ICE   detention center that employs them claims it is not responsible for the abuses.

Contractors who were asked to inspect the detention facilities had also been warning the DHS for years about the harm they cause to children.

Customs and Border Protection (CBP) and Immigration and Customs Enforcement (ICE) are two agencies within the Department of Homeland Security (DHS). The former is charged with protecting the nation's 325 ports of entry and enforcing hundreds of laws designed to protect citizens and commerce, and the later is responsible for enforcing immigration laws within the remaining areas of the U.S. In addition to investigating, arresting, and detaining illegal aliens, ICE is also responsible for the removal of aliens.

According to The Guardian, ICE has nine facilities available to detain children who have been separated from adults, including family members, or who were unaccompanied when they crossed the border. The average length of stay for a child ranges from 100 to 240 days, which exposed them into a very vulnerable situation with no legal representation.

Outgoing Homeland Security Secretary Kirstjen Nielsen once said officials would not apologize for enforcing immigration laws that result in the separation of children from their parents. Then House Minority Leader Nancy Pelosi called the policy "a heartbreaking, barbaric issue" and said it should be ended immediately.

(With input from Reuters.)