PLAYAS DE TIJUANA, Mexico, May 4, 2024 (BSS/AFP) - Mexican authorities said Friday they were intensifying a search for two Australians and an American who went missing last week during a surfing trip in an area beset by cartel violence.

Australian brothers Jake and Callum Robinson and their American friend Jack Carter have not been seen since April 27.

They were on vacation near the coastal city of Ensenada in the northwestern state of Baja California.

But the brothers' mother Debra Robinson said in a Facebook post that they never arrived at their planned accommodation.

Navy personnel were participating in the search along with officials from the state prosecutor's office, which requested help from firefighters to search a cliff in the area, according to city hall.

A source from the prosecutor's office told AFP that no human remains had been found, contrary to media reports that three bodies were discovered in the area.

Baja California state authorities said on Thursday that three Mexican nationals were being questioned in connection to the disappearances.

"A white pickup vehicle was located, as well as other evidence," the state prosecutor's office said in a statement.

The investigation was being coordinated with the US Federal Bureau of Investigation and the Australian and US consulates, it added.

Australian Prime Minister Anthony Albanese has described the disappearances as "really concerning."

"We certainly hope that these brothers are found safely, but there is real concern about the fact that they've gone missing," he told Australian television.

Baja California, known for its inviting beaches, is also one of Mexico's most violent states thanks to organized crime groups.

In November 2015, two other Australian surfers, Dean Lucas and Adam Coleman, were murdered and their bodies burned while traveling in the northwestern Mexican state of Sinaloa.