Mexico has not traditionally used security forces to stop undocumented foreign citizens leaving the country for the United States, and photographs of militarized police catching Central American and Cuban women at the border in recent days have met with criticism.

Mexico is trying to curb a surge of migrants from third countries crossing its territory in order to reach the United States, under the threat of tariffs on its exports by U.S. President Donald Trump, who has made tightening border security a priority.

Responding to weekend reports of heavy-handed interventions by the military, Luis Cresencio Sandoval, the head of the Army, said soldiers were needed to back up migration officials in containment operations.

Alongside 6,500 members of the security forces sent to Mexico’s southern border area with Guatemala, where many migrants enter, a larger contingent was in the north, he said.

“In the northern part of the country we have a total deployment of 14,000, almost 15,000 units between the National Guard and the Army,” Sandoval told a regular news conference.

“If we left it completely in the hands of the National Institute of Migration it wouldn’t be possible,” he added. “That’s why we’re providing support, it’s a strategy being pursued on both borders.”