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 Wednesday renewed his threats to close parts of the U.S.-Mexico border, citing the need to stop a “very big caravan.”

Trump tweeted that a migrant caravan comprising more than 20,000 people had already been “reduced in size by Mexico.” Mexican police detained hundreds of Central American migrants in the southern state of Chiapas on Monday.

“Mexico must apprehend the remainder or we will be forced to close that section of the Border & call up the Military. The Coyotes & Cartels have weapons!” Trump tweeted Wednesday.

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A very big Caravan of over 20,000 people started up through Mexico. It has been reduced in size by Mexico but is still coming. Mexico must apprehend the remainder or we will be forced to close that section of the Border & call up the Military. The Coyotes & Cartels have weapons! — Donald J. Trump (@realDonaldTrump) April 24, 2019

The exact size of the caravan has been disputed.

Mexican Interior Secretary Olga Sanchez Cordero has said the group, comprising migrants from El Salvador, Honduras and Guatemala, has more than 20,000 people, but a WhatsApp group that called for people to gather in El Salvador to begin the trek in late March had just over 200 members, according to The Associated Press.

The immigration activist group Pueblo Sin Fronteras, which has accompanied several caravans in Mexico, said “there has never been a caravan of the size that Sanchez Cordero mentioned,” according to the AP.

Later on Wednesday, Trump accused Mexican soldiers involved in a recent incident with U.S. troops along the border of aiding drug cartels, tweeting that he was "now sending ARMED SOLDIERS" to the border.

Cracking down on illegal immigration has been a central tenant for Trump both during his 2016 presidential campaign and since winning the White House.

Ahead of the 2018 midterm elections, Trump repeatedly invoked the specter of a previous caravan to drum up support for more restrictive immigration policies.

U.S. officials used tear gas on a group of migrants in January after they attempted to cross the border in Tijuana.