From the veranda of his farmhouse on the outskirts of this isolated riverside settlement, Gilvan Onofre can hear the helicopters coming, their rotors slicing through the humid Amazon air.

"There is no longer any way of hiding," sighed Onofre, a 70-year-old cattle rancher who moved to the region in the 1970s seeking his fortune and admits to having destroyed huge tracts of rainforest. "Everyone knows that Ibama is photographing us and what we are doing from two metres above." Ibama is Brazil's environmental protection service, the group tasked with monitoring, catching and punishing those responsible for the plunder of the Amazon rainforest.

Boca do Acre, a cattle-ranching town in the deep south of Amazonas state, is one of the new frontlines of the government's war on illegal deforestation. When Onofre arrived here on 23 December 1972, there was hardly a cow to be seen. Boca do Acre was a tiny community of rubber-tappers surrounded by dense jungle accessible only by river.

Since then the landscape has changed beyond recognition. It now has the largest cattle herd in Amazonas and much of the virgin rainforest has gone, replaced with dozens of sprawling cattle ranches, dotted with white zebu cows and the occasional cowboy. "In 30 or so years, we have gone from zero to 400,000 [heads of cattle]," boasted Onofre, president of the local ranchers' association. "Nowadays everybody says we have to preserve the forest. But when we arrived nobody knew we had to protect anything; we had to deforest. We chopped the trees down so we could feed our animals, our cattle."

Amazonas remains by far the best-preserved of Brazil's nine Amazon states, with about 97% of its original forest cover intact. But environmentalists and government officials fear the state's southern limits are becoming a new frontier for deforestation, with ranchers and loggers looking to push north into the untouched forests.

Ibama's president, Curt Trennepohl, admitted last week, in an interview with the Observer, that "aliens" from other states had set their sights on the forests surrounding Boca do Acre and other towns in southern Amazonas. Ibama's intelligence reports suggested that "people with a history of exploitation and involvement in illegal deforestation" were trying to move in, many of them from the lawless state of Pará.

Recent years have brought positive news for defenders of the Amazon region. Between 2004 and 2009, deforestation fell from around 27,000 to 7,600 square kilometres, but last December the country's environment minister, Izabella Teixeira, announced the lowest levels of deforestation on record, with around 6,451 sq km of forest lost between August 2009 and July 2010. The celebrations proved short-lived. By April, satellite images indicated that, while deforestation had continued to fall in some logging regions, among them Pará, it was on the rise in better preserved regions such as Amazonas. The images indicated that deforestation there soared by around 87% between August 2010 and February this year, compared with the same period a year earlier. Much of that was in Boca do Acre. Neighbouring municipalities, such as Lábrea, Apuí and Novo Aripuanã, also showed troubling levels of destruction.

"If the government is not present to revert this process… then it is going to take over Amazonas state," warned André Muggiati, a Greenpeace campaigner based in Manaus. "The government must not allow Amazonas state to become like Pará and Mato Grosso," he added, referring to two states that have been ravished since the 1960s by illegal loggers, ranchers and soy farmers. "The deforestation has to be stopped there."

Authorities say they have ramped up their activities, with 58 anti-deforestation operations this year. A "zero deforestation pact" was recently signed with ranchers in Boca do Acre and Apuí, under which the seizure of cattle reared on illegally deforested land would be suspended if new deforestation stopped. "This has worked better than we had expected," said Trennepohl, claiming that recent months had seen deforestation around Boca do Acre fall away.

A wider Amazon offensive, particularly in the soy-growing Mato Grosso state, was announced last week after satellite images showed a large rise in destruction there. "For us it was not just a surprise but a shock," Trennepohl said of the new data which showed a jump in deforestation of more than 470% between March and April this year. "It was a surprise that can only be the result of a rise in commodity prices overseas." The government would throw its "whole force" at the region, he vowed. More than 500 Ibama operatives have been sent in and around 200 operations are planned. The army will also be deployed.

In Boca do Acre, ranchers such as Onofre believe the measures are helping to slow the destruction. Frequent Ibama raids had made ranchers "reluctant" to illegally clear land on which to expand their herds, said the rancher. "People know that, if they do, the [Ibama] people will come and screw them."

But not everyone is convinced. Some environmentalists are concerned that increased repression in other parts of the Amazon is creating a "safe haven" for those intent on profiting from rainforest destruction in places such as south Amazonas and fear that the coming years could see a new wave of destruction here. Even Audálio Silva de Noronha, head of Boca do Acre's loggers' union, admitted being concerned about the arrival of large-scale logging groups from other parts of the country. "There is a migration of people, not from here, but from Acre [state], from the south of Brazil, people who come to buy land here with this intention," said 49-year-old Noronha, a furniture maker who extracts his living from the forests surrounding Boca do Acre and employs 24 locals in his workshop. "They are not coming to create [environmental] reserves," he said, but to "exploit". "If there is not rigorous action against deforestation, it will rise, yes. We know that human beings don't know any limits. They always want more and more."

Trennepohl said Ibama's intelligence sector had also detected a movement of loggers and ranchers from other Amazon states. "We will contain the haemorrhage," he promised. "This will not happen in south Amazonas because we have this intelligence and we are watching. Any aliens who arrive buying land will be watched."

Locals, however, say repression will only achieve so much. In Boca do Acre, residents believe destruction will continue unless measures are taken to help those who live in the forest support their families without permanently damaging the environment. "Here all we see is repression… weekly visits from Ibama, to arrest, fine, repress and even embargo areas," complained Noronha. "It is very simple to talk about preservation when you are in an office with the air conditioning on or when you are on Copacabana beach in Rio drinking beer."

Recently fined R$12,000 (£4,500) for using undocumented and illegally deforested timber, Noronha said sustainable forest management projects were urgently needed to allow responsible logging. At the moment, he conceded, "everything is illegal". He added: "Our survival has to come from the forest. There is no other way. There are no universities here. There are no factories. If you don't have a government job, you have to claw some kind of survival from the rivers and the forest."

Back on his ranch, Onofre reflected on his hopes that Brazil's congress would approve a controversial bill altering the forest code and reducing the amount of rainforest Amazon landowners had to protect. Opponents argue that changes to the code would effectively hand an amnesty to environmental criminals.

Failing that, Onofre at least has another business to fall back on: a roadside "love-motel", just opposite his cattle farm, called "G-Spot". It was, he joked, the only part of his local empire that environmental officials could not touch. "If they turn up there, they have to pay!"