An indigenous people whose territories are divided by the Brazil-Peru border in the remote Amazon say they are vehemently opposed to oil exploration on the Peruvian side and are prepared to fight companies in order to keep them out.

The Matsés’s main concerns are the potential social and environmental impacts of oil operations on both sides of the border, where they live in the far west of the iconic Javari Valley Indigenous Territory in Brazil and a 490,000 hectare legally-titled community in Peru.

“I don’t want to die contaminated or from some illness transmitted [by a company],” Waki Mayoruna, the head of a village called Lobo, told the Guardian on a visit to the Javari Territory. “If they don’t understand our no means no, there’ll be conflict that’ll lead to people being killed.”

“Our forests and the headwaters could be contaminated,” says Raimundo Mean Mayoruna, president of the General Mayoruna Organization (OGM) in Brazil. “But this isn’t just an environmental issue. It could bring illnesses. The Matsés are afraid this’ll be the death of them.”

The licence to operate on the Peruvian side is held by Canada-based company Pacific Rubiales Energy, whose two concessions, Lot 135 and Lot 137, cover almost 1.5 million hectares and have been estimated to hold almost one billion barrels of oil.

Pacific Rubiales told the Guardian it is not currently operating in either concession, but it has held the licences since 2007 and performed some exploration in Lot 135 in 2012 and 2013 which was due to involve, according to company plans, drilling three wells and conducting seismic tests.

Earlier this month numerous Matsés in Brazil crossed the border into Peru to attend a bi-national, three-day meeting for Matsés from both countries, which was held in a village called Santa Rosa and concluded with them “totally rejecting” both concessions.

“We maintain the position of not permitting the oil company to enter,” Cesar Nacua Uakui, from a village called Estirón on the Peruvian side, told the meeting. “It doesn’t matter if they kill us. We must look after our land for our children.”

“We definitely don’t want the oil company,” said Carlos Fasabi Panduro, from San José de Añushi in Peru, who acted as the meeting’s translator on the second and third days. “Everyone will say the same. The decision is unanimous.”

Jorge Pérez Rubio, president of the regional indigenous organisation, ORPIO, to which the Matsés community on the Peruvian side is affiliated, told the meeting he “welcomed” their decision and said, “We’re going to avoid the kind of disaster that has happened elsewhere.”

The Matsés issued a statement ratifying their decision and alluding to “serious social and environmental contamination” caused by oil operations in other parts of Peru, and the “continued use of contaminating practices” and “little will to remedy environmental damages.”

They have made similar statements regarding the concessions every year for the last five years after previous bi-national meetings, leading one woman, Delia Rodriguez Lopez, to tell this year’s gathering, “We don’t want to keep talking about this. Our position is no. It’ll be no until the end.”

Numerous Matsés told the Guardian, during visits to villages in both Brazil and Peru after the meeting, that they were prepared to fight oil company personnel – with spears, bows and arrows – if they turned up in their territories.

“They should respect indigenous peoples’ rights, but in my view they’re not doing so,” says Lorenzo Tumi, from Puerto Alegre in Peru. “The only weapon we have is to kill one of them. They have their powers – we have our powers too.”

“We’ll always fight against the invasion of our territories,” says José Tumi, from Sao Meireles in Brazil. “If they don’t listen, we could fight like we did before, with bows and arrows. We could attack anyone who invades our territory. We’re not afraid of dying.”

Last year a delegation of indigenous peoples from the Javari Territory in Brazil, including Marubos, Matís and Kanamaris as well as Matsés, travelled to Brasilia to express their concern about Lot 135 and Lot 137 to Brazil’s Foreign Ministry and Peruvian government representatives.

Two months before, the federation representing indigenous peoples in the Javari Territory, UNIVAJA, released an open letter requesting the suspension of all oil activity bordering the reserve and saying they did not accept “the Peruvian government’s oil projects in the River Yaquerana region.”

“[They] may affect traditional Matsés territory, along the Brazil-Peru border, and isolated indigenous peoples in both countries by polluting headwaters and the River Javari itself,” the letter states. “[This] will not only affect indigenous peoples, but other settlements living along the river.”

OGM is urging Brazil’s Foreign Ministry to establish dialogue with the Peruvian government in order to suspend both Lot 135 and Lot 137 because of threats to natural resources and indigenous peoples’ territories on the Brazilian side of the border.

“The Matsés have lived for a long time in the River Yaquerana region – long before the border was imposed by the Brazilian and Peruvian governments,” OGM stated to the Ministry. “Anything affecting the Matsés on the Peruvian side affects us [on the Brazilian side], and vice versa.”

According to Peruvian NGO CEDIA, Lot 137 includes 49% of the Matsés’s titled community land in Peru and 36% of a supposedly “protected natural area” called the Matsés National Reserve, which they consider their territory too.

Lot 135 also includes areas the Matsés consider their land and, in addition, is superimposed over a proposed reserve for indigenous people living in what Peruvian law, indigenous organisations and anthropologists call “isolation” or “voluntary isolation” (IPVI).

The eastern boundary of Lot 135 and part of Lot 137 is the River Yaquerana – which is the principle source of the River Javari, acts as the Brazil-Peru border, and is used by the Matsés in both countries for drinking, cooking, washing, bathing and fishing.

“The River Yaquerana is the source of life for us,” says Celina Pue, from Puerto Alegre in Peru, which is located on the boundary between the two concessions. “What will happen to us if the river is contaminated? It will be the end.”

Many Matsés are also opposed to Lot 135 and Lot 137 because of the IPVI living upriver on both sides of the border who have almost no interaction with outsiders and could be decimated by contact with oil company personnel.

Numerous Matsés expressed concern to the Guardian and during the bi-national meeting about the potential impacts on the IPVI, and their statement calls for the “immediate termination” of operations in both concessions because of the threats to their lives and territory.

During the meeting in Santa Rosa the president of the Matsés community in Peru, Daniel Vela Collantes, revealed that Perupetro, the government body responsible for establishing oil concessions and contracting companies, had recently requested a meeting with the Matsés.

The decision was taken to invite Perupetro representatives to Matsés territory in order to inform them that they are totally opposed to both Lot 135 and Lot 137 – an invitation that Perupetro subsequently rejected, according to Vela Collantes.

Asked by the Guardian if it would respect the Matsés’s decision, Pacific Rubiales stated, “[It] fully respects the desire of the Matses community of Peru to not have oil operations in their territory, therefore the company is not performing any exploration activities in Block 135 and 137.”

Pacific Rubiales refused to answer questions on the Matsés’s declared preparedness to fight, opposition from other indigenous peoples on the Brazilian side, and whether it accepts IPVI would be impacted by operations in Lot 135. Perupetro could not be reached for comment.