A number of men on horseback with "Police U.S. Border Patrol" emblazoned on their clothing were seen blocking the path of migrants scrambling up the U.S. embankment carrying plastic bags and cardboard boxes.
As the migrants try to get supplies from the Mexican state of Coahuila, U.S. officials have stepped up security at the border and started flying migrants out of the area, some to Haiti.
Migrants said their squalid encampment under a bridge on the U.S. side of the river was short of supplies.
U.S. officials over the last few days had let migrants cross back and forth at a shallow point of the river.
On Sunday, however, they told migrants they would not be able to return to the U.S. side if they ventured into Mexico.