President Donald Trump has threatened negotiations over the North American Free Trade Association (NAFTA), as well as foreign aid to Honduras – one of the poorest countries in Latin America – over a “caravan” containing roughly 1,000 migrants that is supposedly headed for the U.S. border.

“The big Caravan of People from Honduras, now coming across Mexico and heading to our ‘Weak Laws’ Border, had better be stopped before it gets there,” wrote President Trump, referring to a caravan of immigrants reportedly traveling towards the U.S., organized by a humanitarian volunteer group called Pueblos Sin Fronteras, or People Without Borders.

Pueblos Sin Fronteras has organized similar caravans in the past, including two last year. The caravans raise awareness of the plight of migrants and refugees who take the often perilous route through Mexico to the southern border of the United States, while also attempting to make their dangerous journeys safer.

This year, the caravan has been heavily covered by U.S. media, including Fox News – a favorite of the President.

In his tweet posted Tuesday morning, Trump added that “cash cow NAFTA,” as well as “foreign aid to Honduras,” was “in play” if the caravan reached the U.S. border.

“Congress MUST ACT NOW!” he added.

Honduras received $152 million in foreign aid from the United States in 2016, according to U.S. Aid. The Central American country of 9.1 million people has a per capita GDP of $5,725 (about one tenth that of the United States), making it the second-poorest nation in the Western Hemisphere, according to the International Monetary Fund.

In a string of tweets posted Sunday and Monday, President Trump suggested the immigrants in the caravan were attempting to take advantage of the U.S.’ Deferred Action for Childhood Arrivals program, which they would not qualify for.

“Honduras, Mexico and many other countries that the U.S. is very generous to, sends many of their people to our country through our WEAK IMMIGRATION POLICIES,” he wrote on April 2. “Caravans are heading here. Must pass tough laws and build the WALL. Democrats allow open borders, drugs and crime!”

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Write to Kate Samuelson at kate.samuelson@time.com.