HUIXTLA, Mexico—Even as a caravan of thousands of Honduran migrants makes its way across southern Mexico, another caravan is forming in its wake.

Thousands of Honduran migrants gathered in a Guatemalan city near the border with Honduras Tuesday to prepare for a new caravan that would follow in the footsteps of a larger group currently marching to the U.S.-Mexico border, posing a fresh challenge to Guatemalan and Mexican authorities seeking to contain a surge in mass migration.

Church-run charities assisting migrants and activists say as many as 2,500 Hondurans who crossed into Guatemala in recent days have gathered in the city of Chiquimula, near the border with Honduras. But estimates of the size of the new group vary widely, from that number down to a few hundred, according to Francesca Fontanini, spokeswoman for the Americas region for the United Nations office on refugees.

The migrants say they plan to head to Ciudad Tecun Uman, the Guatemalan border town that was overwhelmed by a larger group of migrants who rushed into Mexico over the weekend.

Some 5,000 migrants are already marching northward in Mexico’s southern Chiapas state, according to Mexican authorities, while close to 1,700 Hondurans who requested asylum have been sent to a shelter run by Mexican migration authorities in the city of Tapachula, near the border with Guatemala. Hundreds of them traveled with children and babies.