President Trump vowed Monday to cut off or “substantially” reduce aid to three Latin American nations, voicing fresh frustration as a growing caravan of migrants that originated in Honduras continued to make its way toward the U.S.-Mexico border.

“Guatemala, Honduras and El Salvador were not able to do the job of stopping people from leaving their country and coming illegally to the U.S.,” Trump said in one of a string of morning tweets on the subject. “We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them.”

It was not immediately clear what payments Trump was referring to or the extent to which he could act without congressional approval.

Later Monday, in an exchange with reporters before leaving Washington for a rally in Texas, Trump again mentioned the three countries and that the United States gives them “tremendous amounts of money” for nothing.

“Every year, we give them foreign aid,” Trump said. “And they did nothing for us. Nothing.”

Asked about his claim that the caravan includes “criminals and unknown Middle Easterners,” Trump told reporters to go look for themselves.

“Go into the middle of the caravan, take your cameras and search, okay?” Trump said. “You’re going to find MS-13, you’re going to find Middle Eastern, you’re going to find everything. And guess what? We’re not allowing them in our country. We want safety.”

In his morning tweets, Trump also expressed frustration with Mexico’s military and police, saying they appear “unable to stop the Caravan” and that he has alerted the U.S. Border Patrol and military to what he termed a national emergency.

Ahead of the Nov. 6 midterm elections, Trump has sought to turn the caravan into a symbol of the larger issue of immigration, which the White House believes can be used to drive up turnout among the Republican base.

Trump made that point explicitly in another of his Monday morning tweets.

“Every time you see a Caravan, or people illegally coming, or attempting to come, into our Country illegally, think of and blame the Democrats for not giving us the votes to change our pathetic Immigration Laws!” Trump wrote. “Remember the Midterms! So unfair to those who come in legally.”

Although Trump blames the Democrats, a pair of immigration bills he had backed failed in the Republican-controlled House amid policy disputes between the GOP’s conservative and more moderate wings.

In 2014, when tens of thousands of unaccompanied minors and families from Central America overwhelmed U.S. Border Patrol stations, Congress authorized a $750 million aid package to the Northern Triangle countries to boost economic growth and improve public safety to help create conditions that would prevent the exodus of migrants.

That hasn’t worked, but experts said the strategy would take up to a decade and require continued investment and oversight under a coordinated strategy that was upended when Trump succeeded President Barack Obama.

In the fiscal year just ended, U.S. aid to Guatemala totaled $83.7 million. Honduras received $58.3 million and El Salvador $50.7 million. All amounts were sharply lower than in the previous years. In the fiscal year that just began, planned foreign aid spending is $69.4 million for Guatemala, $65.7 million for Honduras and $45.7 million for El Salvador.

As of Sunday, the caravan of migrants from Central America had grown to roughly 5,000 people, a massive group that stretched along the main highway in Tapachula, Mexico, for more than half a mile.

Carol Morello and Felicia Sonmez contributed to this report.