Under pressure from US President Donald Trump to slow the surge of Central Americans crossing the border, Mexico promised earlier this month to reinforce its southern border with 6,000 National Guardsmen, but had not previously disclosed the extent of the crackdown on its northern border.

"We have a total deployment, between the National Guard and army units, of 14,000, almost 15,000 men in the north of the country," Defense Minister Luis Cresencio Sandoval said at a press conference alongside President Andres Manuel Lopez Obrador.

Asked whether those forces were detaining migrants to prevent them from crossing, Sandoval replied: "Yes."

"Given that (undocumented) migration is not a crime but rather an administrative violation, we simply detain them and turn them over to the authorities" at the National Migration Institute, he said.

The government has faced criticism over migrant detentions at the northern border since an AFP photographer documented last week how heavily armed National Guardsmen in Ciudad Juarez forcefully stopped two women and a young girl from crossing the Rio Grande river into the United States.

The policy is a shift from previous practice. The Mexican security forces had not typically detained migrants at the US border in the past.

Fleeing chronic poverty and brutal gang violence in their home countries, the Central Americans crossing Mexico mostly lack migration papers.

However, international law protects the right of undocumented migrants to cross international borders to request asylum. And the US courts have upheld their right to do that anywhere along the border, whether or not it is an official crossing.

Trump wants Mexico to do more to stop a recent increase in migrant arrivals.

Last month, he threatened to impose tariffs on all Mexican goods, extracting a promise from Lopez Obrador's government to reinforce Mexico's southern border and expand its policy of taking back migrants while the US processes their asylum claims.

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