Vast swathes of the western Amazon are to be opened up for oil and gas exploration, putting some of the planet's most pristine and biodiverse forests at risk, conservationists have warned.

A survey of land earmarked for exploration by energy companies revealed a steep rise in recent years, to around 180 zones, which together cover an area of 688,000 sq km, almost equivalent to the size of Texas.

Detailed mapping of the region shows the majority of planned oil and gas projects, which are operated by at least 35 multinational companies, are in the most species-rich areas of the Amazon for mammals, birds and amphibians.

Researchers used government information on land that has been leased to state or multinational energy companies over the past four years to create oil and gas exploration maps for western Brazil, Peru, Ecuador, Bolivia and Colombia. The maps showed that in Peru and Ecuador, regions designated for oil and gas projects already cover more than two thirds of the Amazon. Of 64 oil and gas regions that cover 72% of the Peruvian Amazon, all but eight were approved since 2003. Major increases in activity are expected in Bolivia and western Brazil.

"We've been following oil and gas development in the Amazon since 2004 and the picture has changed before our eyes," said Matt Finer of Save America's Forests, a US-based environment group. "When you look at where the oil and gas blocks are, they overlap perfectly on top of the peak biodiversity spots, almost as if by design, and this is in one of the most, if not the most, biodiverse place on Earth."

Some regions have established oil and gas reserves, but in others, companies will need to cut into the forest to conduct speculative tests, including explosive seismic investigations and test drilling. Typically, companies have seven years to explore a region before deciding whether to go into full production.

"The real concern is when exploration is successful and a zone moves into the development phase, because that's when the roads, drilling and pipelines come in," said Finer.

Writing in the journal PLoS One, Finer and others from Duke university in North Carolina and Land is Life, a Massachusetts-based environment group, call for governments to rethink how energy reserves in the Amazon are exploited.

One issue, the authors argue, is that while companies must submit an environmental impact assessment for their project, these are often considered individually instead of collectively. "They're not looking at the bigger picture of what happens if there are lots of projects going on at the same time.

"You could have each individual company thinking they're being relatively responsible and keeping their own road networks under control and so on, but what happens when you have 15 other projects around you? All of a sudden, when you look at the bigger picture, you have a sprawling road network," said Finer.

The creation of widespread road networks will put previously inaccessible forest at risk of deforestation, illegal hunting and logging, the authors argue.

The researchers urge companies to adopt a moratorium on new road building, and instead use helicopters to ferry personnel and machinery to and from the sites, as has been done in some locations. They also call for governments to take a broader view of the environmental impacts of new projects, by assessing them as a group rather than individually.

Further research by the team found that many of the planned exploration and extraction projects were on land that is home to indigenous people, who whilst being consulted, have no say in whether a project goes ahead or not. At least 58 of the 64 regions in Peru are on land where isolated communities live, with a further 17 infringing areas that have existing or proposed reserves for indigenous groups.

"The way that oil development is being pursued in the western Amazon is a gross violation of the rights of the indigenous peoples of the region," said Brain Keane of Land is Life. "International agreements and inter-American human rights law recognise indigenous peoples have rights to their lands, and explicitly prohibit the granting of concessions to exploit natural resources in their territories without their free, prior and informed consent," he added.

The report adds that the international community should pay countries in the Amazon to leave forest lands untouched. Ecuador has said it will not develop its largest untapped oil reserve if it receives compensation by the end of the year, an offer that countries have yet to take them up on.