Mexico has protested to the Vatican over purported comments by Pope Francis worrying about a possible "Mexicanisation" of Argentina amid increasing drug trafficking activity in his homeland.

Foreign secretary Jose Antonio Meade said he had met Christophe Pierre, the Vatican's diplomatic emissary to Mexico, to inform him about the protest note.

"We express sadness and concern with respect to the communications that apparently were made, referring to a private letter from Pope Francis," Mr Meade said.

The pontiff apparently touched on drug activity in Argentina in a weekend letter to Argentine MP Gustavo Vera.

"Hopefully we are in time to avoid Mexicanisation," the letter said. "I was talking to some Mexican bishops, and it is terrifying."

Mr Meade said Mexico repeatedly demonstrated its commitment to fighting the cartels and there were more productive things to do than "seeking to stigmatise" the country.

Mr Vera said the Pope "loves the Mexican people very much," but was concerned Argentina did not end up following a path of "undeclared civil war between cartels".

Vatican spokesman the Rev Federico Lombardi would not confirm or deny the letter's authenticity, in line with long-standing policy of not commenting on what the Holy See considers personal or pastoral communications by the Pope.

Parts of Mexico have been plagued by drug violence for years and cartels have extended their operations to other countries in the hemisphere and around the world.