Pressure has mounted on Brazil's government to redouble efforts to find a British journalist and Brazilian indigenous expert missing in the Amazon, as the search entered its fourth day and a suspect was reportedly detained.

High-profile figures including football legend Pele and current star Richarlison led calls for the authorities to get to the bottom of what happened to Dom Phillips, 57, and Bruno Pereira, 41, who disappeared early Sunday in the remote Javari Valley, near Brazil's border with Peru.

President Jair Bolsonaro's government faces accusations of failing to scale up the search fast enough in the far-flung region, which has seen a surge of illegal fishing, drug trafficking and other invasions of protected indigenous lands in recent years.

Brazil insisted it was reinforcing the operation, as local media reported that a suspect had been detained.

A man accused of threatening indigenous inhabitants searching for the missing men was arrested on suspicion of involvement in the disappearance, O Globo newspaper reported.

Police in the northern state of Amazonas said they had questioned five people in all, one of whom was a suspect, without giving further details.

Indigenous activists say Mr Phillips and Mr Pereira received threats the week they disappeared.

Mr Pereira, formerly the head of Brazilian indigenous agency FUNAI's operations in the region, has been a frequent target of death threats for his work fighting illegal logging, mining and poaching.

"We have reinforced the search operation since yesterday," Justice Minister Anderson Torres wrote on Twitter.

The navy said it had sent a helicopter, boats and a jet ski to assist, and the army said it had deployed 150 jungle operations specialists and an additional helicopter.

The case has drawn urgent appeals from leading media organisations and environmental and human-rights groups - joined by a growing list of high-profile figures.

"The fight for the preservation of the Amazon Forest and the protection of indigenous groups belongs to all of us," Pele posted on Instagram.

"I join the many voices that make the appeal to intensify the search and to find them as soon as possible," added the 81-year-old footballer, considered by many the greatest of all time.

Brazil and Everton striker Richarlison tweeted the same video.

"I ask the authorities, please, act urgently and do everything possible to find Dom Phillips and Bruno Pereira!" he wrote.

The plea also reached the Summit of the Americas in Los Angeles, where US President Joe Biden is meeting regional leaders this week, including Mr Bolsonaro.

Activists mounted a giant screen on a truck that stopped at various landmarks, including the iconic Hollywood sign, with the message: "Where are Dom & Bruno?"