Brazil's government has vowed to make every effort to find a British journalist and Brazilian indigenous expert who went missing deep in the Amazon after receiving threats, as distraught relatives warned that every minute counts.

Veteran freelance journalist Dom Phillips, 57, and respected indigenous specialist Bruno Pereira, 41, were last seen early on Sunday.

They were travelling by boat in the remote Javari Valley, an indigenous reservation near Brazil's border with Peru.

It is believed Mr Phillips, who contributed to RTÉ's News at One in 2013, was researching a book.

They were expected back in the small city of Atalaia do Norte by around 9am on Sunday, but never arrived.

This led the authorities to open an investigation and the military to deploy a search operation.

The Brazilian government expressed its "grave concern" in a statement.

"Federal police were immediately mobilised and are taking all possible measures to find (the men) as quickly as possible," it said.

"The Brazilian government will continue supporting the search operation with the urgency the case demands.

"If the disappearance turns out to be the result of criminal activity, all steps will be taken to bring those responsible to justice."

Investigators brought two people in for questioning yesterday, believed to be among the last to have been in contact with the missing men, police said in a statement.

Neither was detained, it said.

Indigenous rights groups working in the area say the men had received threats last week.

Dom Phillips on a previous trip to the Amazon region in 2019

Mr Pereira, an expert currently on leave from Brazil's indigenous affairs agency, FUNAI, who knows the region well, has regularly received threats from poachers, loggers and miners trying to invade isolated indigenous groups' land.

As the 48-hour mark passed with no news, the men's families urged the authorities to act quickly.

"Time is a key factor in rescue operations, particularly if they are injured," Mr Pereira's family said in a statement

It said his partner, three children and other relatives were in "anguish".

Mr Phillips's sister Sian posted a video message online, fighting back tears.

"We are really worried about him and urge the authorities in Brazil to do all they can to search the route he was following," she said.

"Every minute counts."