MEXICO CITY (Reuters) - Mexico has deployed almost 15,000 soldiers and National Guard in the north of the country to stem the flow of illegal immigration across the border into the United States, the head of the Mexican Army said on Monday.

FILE PHOTO: Members of Mexico's National Guard detain Cuban migrants after they were trying to cross illegally the border between the U.S. and Mexico, in Ciudad Juarez, Mexico June 21, 2019. REUTERS/Jose Luis Gonzalez/File Photo

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.”

A new militarized police force formed from soldiers, marines and federal police, the National Guard is at the heart of Mexican President Andres Manuel Lopez Obrador’s plan to restore order in a country convulsed by record levels of violence.

The force is still taking shape, and due to be headed by a retired general under the aegis of the security ministry.

Reuters images taken on Friday showed National Guard members detaining Cuban and Central American women trying to illegally cross from Ciudad Juarez, Mexico into El Paso, Texas.

Former Mexican national security official Gustavo Mohar said Mexico’s security forces had not been used this way before, describing the development as “sad.”

Mohar blamed the change on Trump’s threats to impose tariffs. The National Guard should ideally not be implementing migration policy, he argued, while acknowledging that Mexican migration authorities were overwhelmed.

Mexico on June 7 agreed to reduce significantly the number of migrants reaching the U.S. border within a period of 45 days.

If that fails, Lopez Obrador’s government has said it will consider changing its laws to satisfy Trump’s demand that Mexico become a buffer zone to stop migrants entering the United States.

Most of the people caught on the U.S.-Mexico border are from three Central American countries suffering from high levels of gang violence and poverty: Guatemala, Honduras and El Salvador.

Trump has said he will impose initial tariffs of 5% on all Mexican goods if the migrant flow is not curbed. The tariff could eventually rise as high as 25%, he has said.