Officials said Monday that Cinta Larga Indians have taken a U.N. representative and four other people hostage.
Federal Indian Bureau President Marcio Meira was flying to Roosevelt reservation in the Amazon state of Rondonia to negotiate their release, said a spokeswoman who declined to be identified according to agency policy.
David Martins Castro was identified by the U.N. office in Rio de Janeiro as a member of the High Commission for Human Rights. The Indian Bureau spokeswoman said the Cinta Larga invited his group, which also included a federal prosecutor, to a meeting Sunday, but then prevented them from leaving.
Such hostage takings are a fairly common tactic among Amazon Indians and the situations are usually resolved within hours or days.
Cinta Larga Indians in 2004 massacred 29 wildcat miners who were illegally mining diamonds on their remote reservation, which holds what is believed to be the largest diamond lode in South America.
The massacre ended a four-year-long diamond rush by miners from across Brazil who converged on the 2.7 million hectare (6.7 million acre) reservation.
Indians are demanding that federal police, who are posted at all the main entrances to the reservation, make it easier for Indians to come and go. They also want better health care, education and job opportunities.
Twenty-eight Indians have been charged in the 2004 killings, but the case has stalled over jurisdictional questions about the special status of Indians and their reservations under Brazil's constitution.
Since then, at least two more miners have been killed on the reservation in disputes over money.
Brazilian law bans mining of any kind on Indian reservations, but in recent years, federal authorities have opened some exceptions for Cinta Larga to sell their diamonds.
Under pressure from the Cinta Larga, Brazilian legislators are considering changing the law to allow mining by Indians on reservations. But the move is opposed by some Indian rights group that fear the easy money will destroy the Indians' culture.
Mining companies also are lobbying to be allowed on to reservations, paying the tribes royalties.
The Brazilian Mines and Energy Ministry estimates some US$2 billion (EUR1.4 billion) worth of diamonds have been smuggled off the reservation in the past few years.