President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Tuesday complained that Mexico is not doing enough to curb the flow of migrants into the U.S. and warned he would soon deliver a response.

What that response would be wasn't immediately clear.

The president accused Mexico of "doing virtually nothing" to stop immigrants from heading for the southern border of the United States, suggesting the country is allowing migrants to take advantage of the U.S. immigration system.

"Mexico’s attitude is that people from other countries, including Mexico, should have the right to flow into the U.S. & that U.S. taxpayers should be responsible for the tremendous costs associated w/this illegal migration. Mexico is wrong and I will soon be giving a response!" Trump tweeted.

...Mexico’s attitude is that people from other countries, including Mexico, should have the right to flow into the U.S. & that U.S. taxpayers should be responsible for the tremendous costs associated w/this illegal migration. Mexico is wrong and I will soon be giving a response! — Donald J. Trump (@realDonaldTrump) May 21, 2019

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The president was tweeting as reports were emerging about his plans to appoint former Virginia Attorney General Ken Cuccinelli, an immigration hard-liner, for a new top job on immigration enforcement.

It's unclear what "response" Trump is planning, but the president has periodically blamed Mexico in recent months for failing to adequately curb the flow of migrants toward the U.S.

Trump previously threatened to close parts or all of the southern border if Mexico did not crack down on migrants en route to the U.S., leading lawmakers in both major parties to raise concerns about how that move might damage the economy.

He backed off the idea last month, suggesting Mexico was doing an adequate job of apprehending those headed for the southern border.

Should Trump return to that concept or threaten other economic penalties, it could undo progress made in recent days after the two countries lifted tariffs on one another to clear the path for a new trade deal.

The president last week presented a new immigration plan, which would seek to overhaul the green card system and move toward a "merit-based" process that gives preference to job skills and education rather than family connections. Democrats have declared the proposal "dead on arrival."