The Brazilian Navy dispatched a crew of ten people on Monday to search for British journalist Dom Phillips and Indigenous expert Bruno Pereira.
The pair went missing while reporting in a remote part of the Amazon rainforest near Brazil's border with Peru.
The Javari valley is home to the most uncontacted indigenous people in the world. It is under threat from illegal miners, loggers, hunters and coca-growers.
How is the search operation going?
Navy spokeswoman Cibelly Lopes said the search team would arrive at the isolated base of Atalaia do Norte around 7 p.m. local time (10 p.m. UTC) and then head to the location where the two men were last seen, the riverside town of Sao Gabriel.
Human Rights Watch urged Brazilian authorities to make use of "all available means" to find Phillips and Pereira.
Brazilian presidential frontrunner Luiz Inacio Lula da Silva tweeted "I hope they are found soon (and) that they are safe and well."
Who are the missing men?
Phillips is a freelancer who has written about Brazil for the Guardian, the Washington Post, the New York Times and others. Pereira is one of Brazil's foremost experts on isolated and uncontacted peoples.
Phillips and Pereira were on a reporting trip to the Javari Valley.
Two local Indigenous rights group said in a statement that Phillips and Pereira had received threats the same week they went missing.
Survival International, an NGO that defends tribal peoples, said Pereira had received threats because of his many years of work with indigenous tribes "which makes the need for immediate action to locate him and Dom all the more pressing."
The pair had traveled by boat to Jaburu lake and had been expected to return to the city of Atalaia do Norte by around 9 p.m. on Sunday, the Union of Indigenous Organizations of the Javari Valley (UNIVAJA) and the Observatory for the Human Rights of Isolated and Recently Contacted Peoples (OPI) said.
However, the pair never arrived.
