Pablo Maniguaje, an indigenous Siona man, is wearing a white-sleeved tunic and peccary tooth necklace. To the right, out of sight, is the Putumayo River, running downstream towards Peru and the main trunk of the Amazon.

Maniguaje is talking about his territory. He is keen to emphasise its importance to the Siona people. “The trees, the water, the air … That’s life for us,” he said. “What else is there?”

Behind him stands his village’s meeting room, where a gathering has just finished. Talk was of only one thing: proposals by the Colombian subsidiary of the UK-based company Amerisur Resources to explore for oil on their land. Everyone who spoke was fiercely opposed.

Maniguaje acknowledged it would not be the first time an oil company had operated in Siona territory. Just across from the meeting room stands a health post, now abandoned, dating from the 1990s and donated by Ram Petroleum and Grant Geophysical.

“It was another company then,” Maniguaje said. “They gave us the health post. They cut paths [through the forest to conduct seismic tests].”

It appears to be seismic tests that Amerisur is most interested in, too. The Guardian’s request for a copy of the environmental impact assessment of its proposed exploration was denied by the company on the grounds that it is “private”, but information presented to Maniguaje’s community, Buenavista, during a “prior consultation” process suggests Amerisur wants to cut at least nine seismic lines in order to explore for oil.

But Buenavista is effectively in the way. Amerisur’s concession, Putumayo-12, extends for more than 54,000 hectares (133,000 acres). It entirely overlaps Buenavista’s roughly 4,500-hectare reserve as well as approximately half a 52,000-hectare area into which the Siona are trying to expand the reserve under a government land restitution programme.

Going by the information presented in the consultation, it appears four of the seismic lines would cross the north of the reserve, while another five would go through the potentially expanded area.

Maniguaje, a revered Siona elder, has been playing a key role in Buenavista’s opposition.

Last year, he travelled with Mario Erazo Yaiguaje, the community’s spokesman, to the US for the Global Climate Action Summit, and to the Dominican Republic for a hearing with the Inter-American Commission on Human Rights to speak about the threats posed by Amerisur. In February, he visited Bolivia for another commission hearing.

Like Maniguaje, Erazo is playing a central role in Buenavista’s cause. He described it as “defending our lives and our existence as a people”.

The Siona number roughly 2,600 and were declared at risk of “extermination” 10 years ago by Colombia’s constitutional court. Buenavista, home to about 600 people, is the largest Siona reserve in geographical terms.

“We’re firm in our opposition to the transnationals,” said Erazo. “No company can operate in our territory.”

For almost three years, Buenavista has repeatedly and unequivocally expressed this position. In a public statement in August 2018 to Iván Duque, the Colombian president, and other high-level government representatives, the community alleged Amerisur was violating their collective, territorial and constitutional rights, and endangering their “physical and cultural integrity”.

The Siona repeated similar allegations in a statement in October announcing the death of Felinto Piaguaje Yaiguaje, a widely respected elder.

“We’ve expressed our position on numerous occasions,” said their most recent statement, addressed to Duque and others in January. “Amerisur must abstain from any operations connected to Putumayo-12 in the reserve or area being expanded. We don’t consent to, authorise or permit any extractive activities in our territory.”

Buenavista’s lawyer, Lina María Espinosa Villegas, called the community’s decision “genuinely transcendental”. Based on her interpretation of constitutional court rulings, she argued the Siona effectively have the right to veto Amerisur in the reserve and the potentially expanded area.

In addition, operating in their territory without their consent, Espinosa claimed, would violate the Siona’s rights under international law, as well as instruments such as the UN declaration on the rights of indigenous peoples.

A map of the area showing oil concessions including Putumayo-12 and the Buenavista reserve. Photograph: Map-Digital Democracy

“For a people so tiny, it’s an issue of cultural dignity,” said Espinosa, who works for the international NGO Amazon Frontlines. “It’s a decision of enormous weight and value.”

Buenavista has not always said no, however. Although the consultation ended in 2014 with the Siona and Amerisur reaching no agreements, as company representatives have reportedly acknowledged, the following year, an agreement was struck in March in which the community committed to “facilitating and collaborating” with the company and its subcontractors regarding the exploration. This was in return for various forms of compensation, including 163 million Colombian pesos (£39,000), none of which they ever accepted.

However, according to Erazo and others, they only made this agreement – reluctantly – because an Amerisur representative had verbally given them the impression, through an interpretation of an interior ministry document, that it had already been granted permission to operate.

After Erazo met ministry representatives in Bogotá and discovered no such thing had happened, many Siona felt they had been deceived, and the community reversed its decision.

In April 2016, it issued a public statement – sent to Amerisur – declaring all Siona territory “sacred”, opposing seismic exploration and revoking “any [previous] decision contrary to it”.

The stated reasons for Buenavista’s opposition seem to be myriad.

While the community’s perception of Amerisur is already overwhelmingly negative because of the alleged effects of operations in an adjacent concession just to the west of the reserve, arguably even more important is the potential impact of the proposed exploration on their land, forest and rivers.

For the Siona, these are fundamental sources of food, water, shelter and income, and integral to their culture, spirituality and identity.

Erazo described their decision as a “mandate from our elders”, adding: “We’re thinking of our children, our land, our medicine, our spirituality. It’s for all these reasons we’ve said no to the seismic.”

Equally important is the fear that exploration would further accentuate the armed conflict in Putumayo between the Colombian army, paramilitaries and “guerrillas”, which has engulfed the Siona over the past few decades. To a certain extent, some in Buenavista claim this has already happened as a result of operations in the adjacent concession, Platanillo.

A health post donated by oil companies that previously operated in Siona territory. Photograph: David Hill

One allegation is that the “reconditioning” of the Alea-1 well in the late 2000s led to Farc guerrillas placing landmines in Siona territory as part of an attempt to “repel the army” based at the well, according to a community statement.

Amerisur said it was “disinclined to comment” on the claim.

Another allegation is that at about the same time, mortar bombs fired by the army from a base at a settlement called La Alea inside Platanillo fell into Buenavista’s reserve. This was, the Siona claim, part of military strategy to fight the Farc, keep them away from the oil installations and make it easier for a company to operate. According to Erazo, there were times over a four-year period in which the reserve was shelled “almost constantly” at night.

Arguably no one in Buenavista has suffered more from the armed conflict than two brothers, Celio Yaiguaje Payaguaje and his brother Placido, living in the north of the reserve in a tiny village called Puerto Silencio. Their mother was killed by a Farc landmine in 2012.

Both Celio, Puerto Silencio’s president, and Placido say they are vehemently opposed to exploration in Putumayo-12.

“Guerrilla, paramilitaries, army … they’d all enter,” Placido said. “That’s not what we want. That’s why we’ve said no to the seismic.”

In Buenavista, it appears to be a widespread assumption that Amerisur could not operate without the army, with some Siona believing the company is “protected” by it from the Farc. Soldiers guard platforms and other installations, they say, or travel with oil tankers or other modes of transport.

Recent statements by the community allege the army’s function in the region is “more a way of guaranteeing oil exploitation” than anything else, and claim that although “the peace deal [in 2016 between the Farc and the government] has meant a decrease in exchanges between the Farc and the armed forces, there has still been an increase in military presence and operations to protect the oil infrastructure”. When the Guardian and a Siona delegation approached one Platanillo installation, two armed soldiers immediately emerged.

Erazo claimed: “[The company] needs the army to protect its infrastructure, workers and muleros transporting the crude.”

It is not only the Siona in Buenavista who see it this way. In a campesino (peasant farmer) village called La Rosa, shortly upriver, the local council’s president, Gregorio Rosales, said he believed the army’s main function was to defend oil operations, rather than protect civilians from the armed conflict or fight the “war on drugs”.

“That’s what we’ve been arguing,” Rosales said. “Because before 2012 [the year he says Amerisur arrived around La Rosa], there wasn’t any army here, and the army that is now here … it isn’t to take care of the campesinos or guard the border. It’s to protect the oil infrastructure.”

When asked whether Amerisur has any contract or agreement with the defence ministry to protect its Putumayo operations, the department said six “agreements” have been made since 2008, all with the army specifically, with one signed every year between 2012 and 2017, and the current one due to expire this month.

The aim of the agreements, the ministry said, has been to maintain security in the “general area and area of influence of the company’s activities and operations”.

An Amerisur spokesperson said it “has been operating in the Putumayo region of Colombia for more than 10 years, during which time it has invested heavily both in the region and in supporting the local communities to deliver positive social and economic benefits, while complying with all applicable laws and regulations. The safety of Amerisur’s staff is paramount”.

In addition, numerous Siona in Buenavista allege Amerisur has also reached agreements with the Farc, and it appears to be another widespread assumption that it could not otherwise operate.

The company denies this, but the community made the claim in a report last year following a visit to the reserve by various government agencies. It was made in reference to the prior consultation regarding the proposed exploration in Putumayo-12, which the Siona claim is effectively controlled by the guerrillas.

An Amerisur oil installation in Platanillo, to the west of Buenavista, which the Siona claim has contaminated rivers. Photograph: Mateo Barriga Salazar

“The Farc said that neither the company [Amerisur] nor the community [Buenavista] could proceed with the consultation process without their approval,” the report said, “so they had to have dialogue and establish agreements between the parties.” Various Siona made similar allegations to the Guardian, but none can be named for their own safety.

The Farc have been on the US state department’s list of foreign terrorist organisations since 1997. An Amerisur spokesperson insisted: “It is absurd and outrageous to allege that Amerisur has negotiated with the Farc.”

Despite Buenavista’s obvious opposition, Amerisur’s 2017 annual report suggests it intends to proceed with the proposed exploration. It stated that the prior consultation for Putumayo-12 has been “completed” with Buenavista and two other Siona communities, therefore allowing “further seismic operations to be performed in the block”.

Although it is true that the prior consultation ended with two other Siona communities reaching agreements with the company, this cannot be said about Buenavista. Not in late 2014, or since April 2016, when the March 2015 agreement was revoked.

Espinosa states that under Colombian law, when no agreements are reached during prior consultation, it is the government that “decides if the project goes ahead or not”. In this case, she does not think the correct protocol was followed.

“[That decision] is supposed to be made on the basis of criteria such as objectivity, reasonableness and proportionality, and if negative impacts on indigenous people are possible then corrective measures must be planned for,” she said.

“In this case, that wasn’t done. Given the particular risks to a community like Buenavista, they should have the right to consent or veto.”

Many Siona in Buenavista allege that following the prior consultation and the revocation of the March 2015 agreement, Amerisur has refused to accept their decision and continued to attempt to reach an agreement with them using various underhand strategies.

In its statement in August, the community reported a company representative saying to other indigenous people living in their reserve that, although Amerisur had not yet struck a deal with Buenavista, “that will change shortly”.

Erazo said: “We’ve said no, but that wasn’t sufficient for them [the company] or the government.”

Farc rebels stand in formation in the jungle of Putumayo, Colombia. Photograph: Fernando Vergara/AP

According to the Siona, underhand strategies include the claim that the interior ministry had given Amerisur permission to operate when it had not, attempting to undermine and/or pressure community leaders such as Erazo, and offers of financial incentives to other Siona communities that have reached agreements with the company, on condition that they could convince Buenavista to do the same.

An Amerisur spokesperson said: “No underhand tactics have been used. The ministry of the interior has accompanied all interactions with these communities and has certified proper and due process has been followed.”

Another allegation is of attempting to intimidate Buenavista’s lawyer. According to Espinosa, company representatives filed a criminal complaint accusing her of blocking public access along a Putumayo road in 2017, although she said that was impossible, because on that day, she was visiting installations in Platanillo with high-level government functionaries and more than 30 Siona.

Last year, numerous Colombian and international organisations wrote to the attorney general calling the charge against Espinosa “unfounded” and requesting the investigation be closed.

But Amerisur stands by its accusation. “The community’s lawyer was present at an illegal road blockage affecting the integrity of the company’s personnel and operations,” a company spokesperson said. “Amerisur reported the presence of these persons to local police, as it is required to do by law.”

It is also alleged that Amerisur coordinated a meeting in March 2017 between the UK ambassador to Colombia, Peter Tibber, and purported Buenavista representatives, including one elder in particular who spoke in favour of the company. However, according to community leaders, the elder has not lived in the reserve for various years and he was allegedly offered financial incentives for his remarks.

The following day, Erazo and others spoke to Tibber, it is claimed, and reiterated their opposition to oil operations in their territory.

“Yes, the people [met by ambassador Tibber] are Siona,” a public statement said, “but they are not legitimate authorities of the Buenavista reserve and can’t speak in our name, nor do they have any representative capacity.”

Erazo said Tibber was “surprised” when he heard the Siona he had met the day before were not Buenavista representatives. “I told him I didn’t want to be called by the company any more,” Erazo said, “and after that, they stopped doing so.”

Amerisur denied using underhand tactics regarding the ambassador’s visit. “[Buenavista] was invited to attend the meeting, and those who turned up were given full and open access to the ambassador during his visit,” a spokesperson said.

The situation in the region has become more tense in the past 10 months or so. In August, the Siona reported that preparations for the proposed exploration in Putumayo-12 had already started in both the reserve and the potentially expanded area, with crews from Amerisur and a subcontractor apparently clearing the forest.

In June, they discovered a new road running from one company installation, reportedly intended to cross the potentially expanded area. According to Buenavista’s August statement, “all this was done with the armed forces”.

However, as part of the legal process to expand the reserve, Mario Coral Mejía, a judge at a court in Putumayo, has ordered Amerisur, unless it has Buenavista’s “informed consent”, to “immediately abstain” from any operations connected to seismic tests in Putumayo-12 in the reserve and potentially expanded area until the legal process is completed.

The judge also ordered numerous government agencies, including the national authority of environmental licences, to “suspend or abstain” from granting any licences or permissions connected to oil operations or other natural resource extraction in the same areas.

Coral issued the ruling in August. Amerisur said it has suspended operations but should be able to carry on because, it continues to claim, the company has Buenavista’s consent.

“Amerisur received the informed consent of Buenavista via the prior consultation process in coordination of (sic) the Colombian ministry of interior, which was subsequently reaffirmed through a ‘minutes of agreement’ in March 2015 signed by Buenavista, and therefore the company continued its agreed and approved, limited work programme,” a spokesperson said.

“Amerisur has issued a brief to the civil court outlining the situation and is waiting for confirmation that informed consent had been secured. In the meantime, Amerisur confirms that it has subsequently ceased operations.”

Amerisur repeated the claim that it has Buenavista’s consent in a letter to the community, Duque and others in October.

The letter also rebutted accusations of exacerbating the armed conflict and having links to “terrorists” and other illegal groups, and stated it has suspended operations following Coral’s ruling.

Erazo said Amerisur had stopped operating, but he did not believe the company has Buenavista’s consent. “It’s a con,” he added.

The UK embassy in Colombia could not be reached for comment.