Mexico wants an urgent investigation into how US military-grade weapons are increasingly being found in the hands of Mexican drug cartels, Mexico’s top diplomat has said.

Mexico’s army is finding belt-fed machine guns, rocket launchers and grenades that are not sold for civilian use in the United States.

“The (Mexican) defence department has warned the United States about weapons entering Mexico that are for the exclusive use of the US army,” foreign relations secretary Alicia Barcena said on Monday.

“It is very urgent that an investigation into this be carried out.”

The Mexican army said in June that it had seized 221 fully automatic machine guns, 56 grenade launchers and a dozen rocket launchers from drug cartels since late 2018.

The military-grade US weaponry — which cartels have bragged about and openly displayed on social media — poses a special challenge for Mexico’s army, which along with police and the National Guard already faces cartels operating homemade armoured vehicles and bomb-dropping drones.

In June, defence secretary Luis Cresencio Sandoval said five rocket launchers had been found in the possession of the Jalisco New Generation cartel, four were seized from the rival Sinaloa cartel and three more seized from other cartels.

Mr Sandoval did not specifically say the weapons were from US military stockpiles.

The US government had no immediate response, though the US ambassador to Mexico was expected to speak to the media later on Monday.

While the Mexican army and marines still have superior firepower, the drug cartels’ weaponry often now outclasses other branches of Mexican law enforcement.

Mexico has long had a problem with semi-automatic rifles that are permitted for civilian use in the United States being smuggled into Mexico, where only low-caliber firearms are permitted and strictly regulated.

Mexico has launched legal actions against US arms manufacturers and gun shops, arguing they contribute to violence.

Also on Monday, describing talks last week with US officials, Ms Barcena said the United States is planning to announce sanctions against airlines and transportation companies that move migrants to South and Central America and through Mexico to the US border.

“The United States said it was going to impose sanctions on South American and Central American companies that are transporting migrants irregularly, and they want us to do the same,” Ms Barcena said.

“The (Mexican) Interior Department is going to call on the bus and airline companies, but we don’t want them (the United States) to act unilaterally.”

Mexico, meanwhile, wants changes made to the US CBP One mobile application for asylum-seekers to make appointments.

The app is designed only to work on telephones in northern Mexico, but Ms Barcena said Mexico has asked that coverage be extended to allow appointments to be made from further south, to avoid a pile-up of migrants rushing to Mexico’s northern border cities.