President Trump said in a tweet Monday night that U.S. immigration agents are planning to make mass arrests starting “next week,” an apparent reference to a plan in preparation for months that aims to round up thousands of migrant parents and children in a blitz operation across major U.S. cities.

“Next week ICE will begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States,” Trump wrote, referring to U.S. Immigration and Customs Enforcement. “They will be removed as fast as they come in.”

Large-scale ICE enforcement operations are typically kept secret to avoid tipping off targets. In 2018, Trump and other senior officials threatened the mayor of Oakland, Calif., with criminal prosecution for alerting city residents that immigration raids were in the works.

Trump and his senior immigration adviser, Stephen Miller, have been prodding Homeland Security officials to arrest and remove thousands of family members whose deportation orders were expedited by the Justice Department this year as part of a plan known as the “rocket docket.”

In April, acting ICE director Ronald Vitiello and Homeland Security Secretary Kirstjen Nielsen were ousted after they hesitated to go forward with the plan, expressing concerns about its preparation, effectiveness and the risk of public outrage from images of migrant children being taken into custody or separated from their families.

Vitiello was replaced at ICE by former FBI and Border Patrol official Mark Morgan, who had impressed the president with statements on cable television in favor of harsh immigration enforcement measures.

In his first two weeks on the job at ICE, Morgan has said publicly that he plans to beef up interior enforcement and go after families with deportation orders, insisting that the rulings must be carried out to uphold the integrity of the country’s legal system.

“Our next challenge is going to be interior enforcement,” Morgan told reporters June 4 in Washington. “We will be going after individuals who have gone through due process and who have received final orders of deportation.

“That will include families,” he said, adding that ICE agents will treat the parents and children they arrest “with compassion and humanity.”

U.S. officials with knowledge of the preparations have said in recent days that the operation was not imminent, and ICE officials said late Monday night that they were not aware that the president planned to divulge their enforcement plans on Twitter.

Executing a large-scale operation of the type under discussion requires hundreds — and perhaps thousands — of U.S. agents and supporting law enforcement personnel, as well as weeks of intelligence gathering and planning to verify addresses and locations of individuals targeted for arrest.

The president’s claim that ICE would be deporting “millions” also was at odds with the reality of the agency’s staffing and budgetary challenges. ICE arrests in the U.S. interior have been declining in recent months because so many agents are busy managing the record surge of migrant families across the southern border with Mexico.

The family arrest plan has been considered even more sensitive than a typical operation because children are involved, and Homeland Security officials retain significant concerns that families will be inadvertently separated by the operation, especially because parents in some households have deportation orders but their children — some of whom are U.S. citizens — might not. Should adults be arrested without their children because they are at school, day care, summer camp or a friend’s house, it is possible parents could be deported while their children are left behind.

Supporters of the plan, including Miller, Morgan and ICE Deputy Director Matthew Albence, have argued forcefully that a dramatic and highly publicized operation of this type will send a message to families that are in defiance of deportation orders and could act as a deterrent.

According to Homeland Security officials, nearly all unauthorized migrants who came to the United States in 2017 in family groups remain present in the country. Some of those families are awaiting adjudication of asylum claims, but administration officials say a growing number are skipping out on court hearings while hoping to live and work in the United States as long as possible.

Publicizing a future law enforcement operation is unheard of at ICE. Trump administration officials blasted Oakland Mayor Libby Schaaf last year for warning immigrants about an impending raid, saying she endangered agents’ safety.

“The Oakland mayor’s decision to publicize her suspicions about ICE operations further increased that risk for my officers and alerted criminal aliens — making clear that this reckless decision was based on her political agenda with the very federal laws that ICE is sworn to uphold,” then-ICE Deputy Director Thomas D. Homan said at the time.

Homan later retired, but last week Trump said Homan would return to public service as his “border czar.” On Fox News, Homan later called that announcement “kind of premature” and said he had not decided whether to accept the job.

Schaaf responded late Monday to the president’s tweet teasing the looming ICE roundups.

“If you continue to threaten, target and terrorize families in my community . . . and if we receive credible information . . . you already know what our values are in Oakland — and we will unapologetically stand up for those values,” she wrote.