The Mexican government says two US agents who died last weekend in a car crash had not been authorised to operate in the country.

The officials, who reportedly worked for the CIA, had taken part in a raid on suspected drug labs in the northern state of Chihuahua before their vehicle skidded off a mountain road and exploded.

Following an investigation ordered by President Claudia Sheinbaum, Mexico's security ministry said that "neither had formal accreditation to participate in operational activities" and that federal authorities had not been informed of their presence.

The incident comes amid at-times fraught relations between Mexico and the US on a number of issues including counter-narcotics.

Sheinbaum has come under intensifying pressure from US President Donald Trump to crack down on drug trafficking, but has repeatedly declined offers of US-led counter-narcotics operations in the country.

She has been adamand that foreign officials can only operate on Mexican soil if given prior clearance by the federal government, and has insisted the Latin American nation's sovereignty should not be breached.

Mexico's security ministry said on Saturday that immigration records showed one of the agents had entered the country as a "visitor", while the other had travelled on a diplomatic passport.

"Neither had formal accreditation to participate in operational activities within national territory," the ministry said in a statement, noting that Mexican law prohibits foreign agents from taking part in operations without federal approval.

US media previously reported that the two individuals, who have not been named, had worked for the CIA as part of an expanded effort to combat narcotics trafficking in the region - a key focus of the Trump administration.

The crash occurred on 19 April, when a convoy returning from an operation targeting suspected methamphetamine labs in a remote, mountainous area veered off the road and plunged into a ravine. Two members of the Chihuahua State Investigation Agency were also killed, officials said.

Chihuahua State Attorney General César Jáuregui told a news conference that the US officials were "instructor officers" from the US embassy engaged in "training work as part of the general and normal exchange we have with the US authorities".

However, Sheinbaum said last week that, while intelligence-sharing with Washington was ongoing, there were "no joint operations on land or in the air".

Last September, a Reuters investigation found the CIA had been running covert operations in Mexico for years to track down the country's most-wanted drug traffickers.

With the Mexican government's approval, the CIA has provided select Mexican units with training, equipment and financial support for operations, including travel, according to Reuters.

Sheinbaum has repeatedly said that her government would work with Washington on security issues, while resisting US threats to put boots on the ground.

Her government has recently pursued an aggressive crackdown on drug trafficking to assuage Trump, who has also repeatedly criticised Mexico for undocumented migration across the two's shared border.