This is not about contacting them; they have been isolated and should remain that way

Indian agency official

The team is to travel along several tributaries of the Amazon river, near the border with Colombia and Peru, where monitoring planes have detected indigenous communities in recent years.

They will try to identify roads and huts to demarcate their land and find out if there is any threat of invaders.

An official of the agency said the expedition would try to avoid direct contact with any of the isolated groups.

"This is not about entering into contact with them," Funai official Manoela Mescia Costa said.

"The idea is to find them and then demarcate the territory they occupy. They have been isolated and should remain that way."

He added that the expedition would also try to find out if illegal loggers or miners had been active in the area.

The expedition consists of 20 researchers who will spend about eight weeks travelling some 4,000km through areas of the Amazon basin accessible only by boat.

Funai estimates that 53 Indian tribes live in isolation in Brazil, most in the Amazon forest.

The BBC's Tom Gibb in Sao Paolo says the launch of the expedition coincides with a growing debate in Brazil about official policy towards the country's Indians.

Mineral expoitation

Some senior army officers have criticised plans by Funai and the government to set up new Indian reservations, saying that the policy could limit Brazilian sovereignty.

The army has drawn up ambitious plans to deploy troops along Brazil's Amazon borders, a move it says is necessary to fight drugs and to stop a spillover of the war in Colombia.

Our correspondent says several powerful local politicians, who want to exploit mineral wealth in the Amazon, are also opposed to the reservations .

About 4,000 of the country's 350,000 Indians are thought to live in the area, which is known as the Javari valley.