Mexico City (AFP) - Mexico's president vowed Tuesday to investigate the controversial detention of migrants trying to cross the US border, saying the 15,000 troops he has deployed there have no such orders.

President Andres Manuel Lopez Obrador, a leftist who took office in December vowing to protect migrants' rights, has come under fire over an AFP journalist's images last week of heavily armed National Guardsmen forcibly detaining two women and a girl at the Rio Grande river, across from El Paso, Texas.

International law protects the right of undocumented migrants to cross borders to seek asylum, and Mexico had not typically stopped them from doing so at its northern border.

However, Lopez Obrador is facing pressure from US President Donald Trump to slow a surge of Central American migrants, and his government is eager to show results and avoid the punitive tariffs the US president threatened last month to impose on Mexican goods.

Facing a backlash at home, the leader known as "AMLO" denied the National Guard and army were under orders to stop migrants fleeing violence and poverty from crossing the US border.

"No such order has been issued, and we are going to review that case, so that it doesn't happen again, because that's not our job," he told a news conference.

- Mixed signals -

That statement contradicted what Lopez Obrador's own defense minister said Monday in a joint news conference with the president.

Asked whether Mexican forces were detaining migrants to prevent them from crossing the northern border, Defense Minister Luis Cresencio Sandoval replied: "Yes."

National Guardsmen interviewed Monday by AFP in Ciudad Juarez, across from El Paso, confirmed their superiors had instructed them to stop migrants from crossing the border.

"They tell us we're not detaining enough, that migration levels are the same," said one, speaking on condition of anonymity.

"When they saw the photo (of the migrant detention), they told us we can't touch the migrants. But at the same time, they order us to detain them and produce results," he added.

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Another told AFP the detained migrants sometimes cry and beg to be released.

"But I can't do that. They'll punish me if I do that. I have to (detain them) to do my job, to finish my deployment here and see my family again soon."

- 'Freedom! Freedom!' -

Mexico dodged Trump's tariff threat by agreeing to reinforce its southern border with 6,000 National Guardsmen and expand its policy of taking back migrants while the US processes their asylum claims.

The deal, reached on June 7, gives Mexico 45 days to show results.

Initially, Lopez Obrador created the National Guard -- a new force that is still being set up -- for an entirely different purpose: to fight drug cartels and curb rising levels of violence.

The force's mission creep has caused controversy in Mexico.

"What's happening in terms of migration policy is a disgrace -- the absence of a plan, the improvisation, the lack of a strategy," said researcher Javier Urbano of the Iberoamerican University, underlining that the National Guard "are trained to fight organized crime."

Using the new force to curb migration will just push migrants to seek out "more dangerous, more remote routes" and give human traffickers more business, he told AFP.

As if to make that point, Mexican media published a haunting photograph of the bodies of a Salvadoran man and his two-year-old daughter who drowned trying to cross the Rio Grande on Sunday.

Four more migrants from Guatemala died of apparent heat exhaustion and dehydration trying to cross the border in the desert, officials said Monday.

Highlighting migrants' mounting frustration with Mexico's crackdown, hundreds of Haitian, African and Asian migrants protested at the Century XXI detention center in the southern state of Chiapas, where they are being held, shouting "Freedom! Freedom!" and trying to escape.

National Guardsmen managed to contain the protest.