Reuters cited two sources with knowledge of the investigation as saying Mexican authorities had found three bodies in Baja Peninsula.
Perth siblings Callum and Jake Robinson, both in their 30s, were travelling in the region on a surfing holiday when they failed to check into pre-arranged accommodation near the city of Ensenada.
Their mother Debra Robinson appealed for help to find her two sons and a US friend they were travelling with, saying she had not heard from them since Saturday.
"Callum is a type one diabetic so there is also a medical concern," she wrote on Mexican social media.
Two Perth brothers were driving a ute when they disappeared. (Supplied/AAP PHOTOS)
Baja California state chief prosecutor María Elena Andrade Ramírez said police had questioned three people in relation to their disappearance but the chances of finding the men had diminished as a result of delays to the investigation.
"Unfortunately, a notice of their disappearance was only filed in the last few days, so very important hours were lost there," she told a press conference in Mexico on Friday.
Investigators have found a burnt-out white ute and abandoned tents believed to have belonged to the missing men.
Officials from the Department of Foreign Affairs and Trade have been providing the family with support.
The department has urged people to exercise a high degree of caution when travelling to Baja California "due to the threat of violent crime".
Drug cartels are known to operate in the region and the state's chief prosecutor said "all lines of investigation" remained open.
In 2015, Western Australian surfers Adam Coleman and Dean Lucas were murdered, believed to have been shot by gang members in the neighbouring Sinaloa region, before their van and bodies were burnt.
Australian Prime Minister Anthony Albanese, as well as opposition foreign spokesman Simon Birmingham and WA Premier Roger Cook, expressed their concerns for the surfers' safety and wishes for their safe return.
with Reuters
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