Six migrants died after Mexican soldiers fired on a group of 33 migrants travelling in a pick-up truck that had tried to evade a military patrol, the defence ministry said on Wednesday, underlining tensions on Mexico's southern border as it faces U.S. pressure to contain migration.

Another 10 migrants were injured in the incident on Tuesday evening, the ministry said in a statement. The group included people of Egyptian, Nepalese, Cuban, Indian and Pakistani nationality, though the statement did not specify the nationalities of the deceased.

The ministry said the incident took place just before 9 p.m. local time on Tuesday while the patrol travelled on a highway near the town of Huixtla, some 40 km (25 miles) from Tapachula, by the Guatemalan border.

The pick-up truck was followed by two vehicles similar to those used by criminal groups in the area, it said, and soldiers reported hearing explosions after which two officers opened fire.

Four migrants were killed at the scene while two others died later in hospital.

The deaths shine a renewed spotlight on Mexico's policy towards migrants as well as the growing role of the military in the country's security.

"These events are neither accidental nor isolated, they are a consequence of the restrictive immigration policies that the Mexican state continues to implement," the Collective for the Monitoring of the Southern Border, a grouping of advocacy and civil society organizations, said in a statement.

Mexico has been under pressure by the United States to reduce the number of migrants arriving at their shared border, where record numbers of people have tried to cross in recent years, fleeing economic hardship and violence.

In recent months illegal border crossings have fallen, as the Biden administration has toughened its approach to border security in the run-up to Nov. 5 elections, where Vice President Kamala Harris faces Republican Donald Trump, who has vowed a wide-ranging crackdown if reelected.

Tapachula, in southern Chiapas state, is an entry point into Mexico for many migrants making the arduous journey towards the United States. Chiapas has also seen clashes this year between the Jalisco New Generation Cartel and Sinaloa Cartel. In July, the turf wars sent some 600 people fleeing across the border into Guatemala.

Mexico's defense ministry said the two soldiers who fired on the migrants were removed from their posts and federal prosecutors had been informed. A military tribunal will also carry out its own investigation.

It said it reaffirmed "its commitment to act in strict accordance with the rule of law, under a policy of zero-impunity, and is ready to help the civil authorities to shed light on the facts."

The ministry added it was coordinating with Mexico's foreign ministry to contact corresponding embassies.