The sister of a British journalist missing in the Amazon has pleaded with Brazilian authorities to “do all they can” to find him.

Dom Phillips, 57, and Bruno Araújo Pereira, a Brazilian adviser at the Univaja association of people in the Vale do Javari Indigenous territory, were last seen early on Sunday in the Sao Rafael community, according to the association.

Mr Phillips has been reporting on Brazil for 15 years for several media outlets, including The Guardian, and has been working on a book on the environment with the help of the Alicia Patterson Foundation.

The pair were due to head to Atalaia do Norte, the most western municipality in the Amazonas region, from the Vale do Javari, Brazil’s second-largest Indigenous territory, but never made it.

Speaking to The Guardian, Sian Phillips said: “My brother Dom has been living in Brazil with his Brazilian wife. He loves the country and cares deeply about the Amazon and the people there.

“We knew it was a dangerous place, but Dom really believed it’s possible to safeguard the nature and livelihood of the indigenous people.

“He is a talented journalist and was researching a book when he disappeared yesterday. We’re really worried about him and urge the authorities in Brazil to do all they can.”

Ms Phillips urged authorities to “search the routes he was following” and added: “If anyone can help scale up resources available for the search, that would be great, because time is crucial.

“Here in the UK, my other brother and I are desperately worried. We love him and want him and his Brazilian guide Bruno Pereira found. Every minute counts.”

Univaja said the two had been threatened during their reporting trip, and Mr Pereira regularly carried a gun due to receiving threats from illegal fishermen and poachers, according to the Associated Press.

The pair disappeared while returning from a two-day trip to the Jaburu Lake region, where Mr Phillips interviewed local Indigenous people, Univaja said.

Brazil’s federal public prosecutors said in a statement on Monday they had opened an investigation and that the Federal Police, Amazonas state’s civil police, the national guard and navy had been mobilised.

The navy, which prosecutors described as co-ordinating the search, said it sent a search-and-rescue team of seven and would deploy a helicopter on Tuesday.

The Vale do Javari region has experienced repeated shootouts between hunters, fishermen and official security agents, who have a permanent base in the area.