WASHINGTON (Reuters) - President Donald Trump said on Monday that U.S. authorities would begin next week removing millions of immigrants who are in the United States illegally.

"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 said. 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.

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."

U.S. 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.

(Reporting by Eric Beech; Editing by Mohammad Zargham and Peter Cooney)