Sao Paulo, Brazil - Prosecutors are investigating a reported massacre in which members of an "uncontacted" indigenous tribe were allegedly killed by illegal gold miners in a distant part of the country's Amazon.

The news has been widely condemned by advocacy groups that say Brazil is backsliding on its obligation to protect indigenous peoples and their land.

"The government doesn't see protecting indigenous people as a priority," said Gilderlan Rodrigues, a missionary working with uncontacted peoples for Brazil's Indigenous Missionary Council, an organisation that monitors violence against indigenous people.

"They want to open up indigenous lands for exploration," he added.

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The killings reportedly happened around the Jandiatuba River, in the Vale do Javari, in Amazonas state, near the border with Peru, an area known as the "uncontacted frontier" that concentrates the highest number of uncontacted tribes - indigenous groups that have no contact with modern society - in the world.

Pablo Luz de Beltrand, the federal prosecutor in charge of the case, confirmed to Al Jazeera an investigation was under way and said a complaint was registered by Brazil's National Indigenous Foundation (Funai) at the beginning of August. The prosecutor could not comment on any details of the case.

A document obtained by Al Jazeera, said to be a copy of the original complaint made by Funai, said about 10 indigenous people - known as "flecheiros" or "archers" and including women and children - hunting for turtle eggs by the edge of the river were killed by illegal gold miners searching for food.

In a note sent to Al Jazeera, Funai confirmed the incident was reported after gold miners were overheard in a nearby town talking about the killings and were arrested and taken in for questioning.

Funai said the deaths were yet to be confirmed and "to date, no material evidence has been found to substantiate the alleged massacre, so it is not possible to confirm the veracity of the deaths".

"In this kind of situation, when indigenous massacres occur in the Amazon, given the time that passes and the size of the area, bodies are rarely found," said Felipe Milanez, a professor of Decolonization of Knowledge, Society and Environment at Brazil's Federal University of Reconcavo da Bahia and researcher of indigenous conflicts.

The 85,445-square-kilometre indigenous territory is roughly the size of Austria and was officially demarcated - the process by which indigenous people have the legal protections to their land - in 2001, in theory, giving them greater protection.

In recent years, however, the presence of illegal miners and loggers has increased in the region, as well as drug traffickers utilising the waterways to import cocaine from Peru as Brazil's internal market has grown, as has its importance as an international transhipment destination.

Funai said at the end of August that there had been an operation to combat illegal mining in the region, led by Brazil's environmental police and army, during which four aquatic mining dredgers were destroyed and six illegal miners fined $320,000.

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Beltran confirmed that an illegal mining investigation had been ongoing since 2014 and there are two open investigations into killings of uncontacted indigenous people in the region, the first reported in February.

Experts attribute the uptick in illegal activity and associated violence to insufficient surveillance. Since 2014, of the four monitoring bases in the region, one has closed, and the remaining three are barely operating, according to Funai.

"The government doesn't have the presence to protect this region, it's an easy passage for illegal activity," said Rodrigues, adding complaints of illegal mining and violence had been made since 2014.

"Unfortunately, the voice of the indigenous was not listened to by the Brazilian government," he said.

Paulo Marubo, president of Union of Indigenous Peoples of the Javari Valley, blamed the violence directly on federal government funding cuts to Funai - which operated the monitoring posts - and said without increased surveillance there would likely be more conflict.

"Without Funai staff, indigenous people are advised not to do surveillance because they do not have the minimum of conditions to do so," he said, referring to indigenous volunteers manning monitoring posts in the absence of Funai staff.

"This guidance is due to confrontations with invaders from when the posts still had the resources to operate," Marubo added.

Funai confirmed by email it suffered a 44 percent funding cut earlier this year, further affecting the possibilities of effective surveillance in the region.

Advocacy group Survival International first warned that the cuts to the Funai bases in April would have disastrous consequences for uncontacted tribes in the Amazon.

"This reported massacre is the latest illustration of the danger that uncontacted tribes face when their land is not protected," said Sarah Shenker, a senior campaigner with the London-based rights group. "They simply won't survive without their land."

The reported massacre would be the second deadliest after 16 Yanomami indigenous people were killed in Roraima state in 1993 by illegal gold miners.

Milanez said Brazil's economic crisis and high unemployment were leading more people going to try their luck with illegal gold mining. "This is happening all across the Amazon," he said.

Indigenous and environmental groups have warned indigenous rights have been increasingly threatened since President Michel Temer took over from Dilma Rousseff in 2016.

Critics accuse the president, who recently survived a congressional trial for corruption and will shortly face another, of acting at the behest of powerful agriculture and mining interests.

"The president is clearly acting in line with his allies in congress who represent agriculture and mining interests," said Rodrigues.

Temer recently signed a decree to open up a vast Amazon reserve, including two indigenous territories, but that was suspended by a federal judge following outrage by environmental groups.

Another ruling enacted by Temer set to implement time limits on indigenous territories, recommending only lands occupied before Brazil's 1988 constitution was drawn up would be considered held by indigenous people.

In June this year, United Nations and inter-American experts warned indigenous and environmental rights were under attack in Brazil, a month after members of the Gamela tribe in Maranhao were attacked with machetes and rifles in a land dispute.

In April, thousands of indigenous people marched in the capital, Brasilia, demanding land rights and protesting against encroaching loggers and miners.

Follow Sam Cowie on Twitter: @samcowie84