President Donald Trump said on Monday that U.S. authorities will begin next week removing millions of illegal immigrants in the United States.

'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 tweeted, referring to the Immigration and Customs Enforcement agency. 'They will be removed as fast as they come in.'

He did not offer specifics.

There are an estimated 12 million immigrants who are in the United States illegally, mainly from Mexico and Central America.

President Trump (seen at the White House on Friday) announced on Monday that Immigration and Customs Enforcement (ICE) will 'begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States'

'They will be removed as fast as they come in,' the president said in his tweet

Trump also said in the tweet that Guatemala 'is getting ready to sign a Safe-Third Agreement.'

Under a deal reached earlier this month, Mexico has agreed to take Central American immigrants seeking asylum in the United States until their cases are heard in U.S. courts.

The agreement, which included Mexico pledging to deploy National Guard troops to stop Central American immigrants from reaching the U.S. border, averted a Trump threat to hit Mexican imports with tariffs.

Trump also said in the tweet that Guatemala 'is getting ready to sign a Safe-Third Agreement.'

A 'safe third agreement' is an agreement in which a country commits to resettling refugees if they first set foot on their soil. This is designed to prevent migrants from settling in their preferred final destination.

The U.S. has only one 'safe third agreement', and that is with Canada. That agreement was signed in 2004.

Vice President Mike Pence suggested last week that Guatemala could receive asylum seekers from its neighbors as a so-called safe third country.

Details of the plan have not been made public, and Guatemala has not publicly confirmed talks that the U.S. State Department said were taking place in Guatemala on Friday.

U.S. rights group Human Rights First said, however, it was 'simply ludicrous' for the United States to assert that Guatemala was capable of protecting refugees, when its own citizens are fleeing violence.

Mexico has agreed that if its measures to stem the flow of migrants are unsuccessful, it will discuss signing a safe third country agreement with the United States.

Last month, it was reported that the administration considered a mass roundup and deportation of thousands of migrant families who have settled in some of America's largest cities. An ICE agent is seen above in this 2015 stock image

Last month, it was reported that Homeland Security officials considered arresting thousands of migrant families who had final deportation orders and removing them from the U.S. in a flashy show of force, but the idea was tabled as the Trump administration grappled with straining resources and a growing number of Central Americans crossing the border.

The idea was to arrest parents and children in 10 cities with large populations of immigrants living in the U.S. illegally, specifically New York, Los Angeles and Chicago, they said, without naming others.

The proposal, first reported by The Washington Post, was meant to send a message and possibly deter others from coming across the border, they said.

But then-Immigrations and Customs Enforcement head Ron Vitiello and then-Secretary Kirstjen Nielsen put the proposal aside over concerns about diverting resources from the border, a lack of detention space and the possibility of renewed public outrage over treatment of families.

The Trump administration separated children from parents at the southern border last summer, a move that prompted mass outrage and criticism that the U.S. was abandoning its humanitarian role and harming children.

Immigration experts say the separations, which were halted last June, did little to stop migrant crossings and, in fact, may have prompted more people to come.

Trump posted the tweet on the same day it was announced that his administration is easing previously announced cuts in hundreds of millions of dollars in aid to the Central American nations of El Salvador, Honduras and Guatemala but will not allow new funding until those countries do more to reduce migrant flows to the United States.

The State Department said that after a review of more than $615million in assistance that President Trump ordered in March to be cut entirely, it would go ahead with $432million in projects and grants that had been previously approved.

The remaining amount will be held in escrow pending consultations with Congress, it said.

Migrants scramble across the Rio Bravo to surrender to the American authorities, on the US-Mexico border between Ciudad Juarez and El Paso on Saturday

That $432million, which comes from the 2017 budget, is being spent on health, education and poverty alleviation programs as well as anti-crime efforts that many believe help reduce migrant outflows from the impoverished Northern Triangle region.

About $370million in money from the 2018 budget will not be spent and instead will be moved to other projects, the State Department said.

'Previously awarded grants and contracts will continue with current funding,' department spokeswoman Morgan Ortagus said.

She added that assistance 'to help the Northern Triangle governments take actions that will protect the U.S. border and counter transnational organized crime will also continue.'

U.S. officials said the review looked at roughly 700 projects funded with fiscal 2017 money by the United States in the three countries and concluded that a significant number were too far advanced to end them.

Trump's decision in March to cut all direct aid to El Salvador, Honduras and Guatemala over the migration issue elicited harsh criticism from Congress where lawmakers from both parties said the assistance was key to helping improve conditions in the three countries that have contributed to the people leaving.

Lawmakers are also expected to object to the latest announcement, which comes as Trump has ratcheted up pressure on Mexico and its southern neighbors to drastically reduce the numbers of migrants heading to the U.S.

Ortagus told reporters the administration was leaving the door open to future funding but would first have to see progress on migration.

'We will not provide new funds for programs in those countries until we are satisfied that the Northern Triangle governments are taking concrete actions to reduce the number of migrants coming to the U.S. border,' she said.

'This is consistent with the president's direction and with the recognition that it is critical that there be sufficient political will in these countries to address the problem at its source.'