Developing countries have reacted angrily to revelations that the United States spied on other governments at the Copenhagen climate summit in 2009.

Documents leaked by Edward Snowden show how the US National Security Agency (NSA) monitored communication between key countries before and during the conference to give their negotiators advance information about other positions at the high-profile meeting where world leaders including Barack Obama, Gordon Brown and Angela Merkel failed to agree to a strong deal on climate change.

Jairam Ramesh, the then Indian environment minister and a key player in the talks that involved 192 countries and 110 heads of state, said: "Why the hell did they do this and at the end of this, what did they get out of Copenhagen? They got some outcome but certainly not the outcome they wanted. It was completely silly of them. First of all, they didn't get what they wanted. With all their hi-tech gizmos and all their snooping, ultimately the Basic countries [Brazil, South Africa, India and China] bailed Obama out. With all their snooping what did they get?"

Martin Khor, an adviser to developing countries at the summit and director of the South Centre thinktank, said: "Would you play poker with someone who can see your cards? Spying on one another like this is absolutely not on. When someone has an upper hand is very disconcerting. There should be an assurance in negotiations like this that powerful players are not going to gain undue advantage with technological surveillance.

"For negotiations as complex as these we need maximum goodwill and trust. It is absolutely critical. If there is anything that prevents a level playing field, that stops negotiations being held on equal grounds. It disrupts the talks," he said.

The NSA would keep US negotiators abreast of their rivals' positions, the document says. "Leaders and negotiating teams from around the world will undoubtedly be engaging in intense last-minute policy formulating; at the same time, they will be holding sidebar discussions with their counterparts, details of which are of great interest to our policymakers … Signals intelligence will undoubtedly play a significant role in keeping our negotiators as well informed as possible throughout the negotiations," it reads.

The document shows the NSA had provided advance details of the Danish plan to "rescue" the talks should they founder, and also had learned of China's efforts to coordinate its position with India before the conference.

The talks – which ended in disarray after the US, working with a small group of 25 countries, tried to ram through an agreement that other developing countries mostly rejected – were marked by subterfuge, passion and chaos.

Members of the Danish negotiating team told the Danish newspaper Information that both the US and Chinese delegations were "peculiarly well-informed" about closed-door discussions. "They simply sat back, just as we had feared they would if they knew about our document," one source told Information.

British negotiators at the summit declined to say whether their negotiating positions had been informed by US intelligence. "It is a longstanding policy that we do not comment on intelligence matters," said a spokesman for the Department of Energy and Climate Change, the UK government department that led the negotiations in Copenhagen.

Ed Miliband, who as energy secretary led the political negotiations for Britain, declined to comment. However, at the time, he was furious that the Danish text which the US had received advance information about, had been leaked to the Guardian.

But one key negotiator for the G77 group of 132 developing countries, who asked not to be named, said at the time that he strongly believed that the US was eavesdropping on his meetings and would only talk in a secure back room that he thought was not bugged. "I was well aware that they seemed to know what our position was before we did," he told the Guardian.

But Ramesh said that he had no idea that the US was spying on him. "I didn't get a sense that I was being followed. I didn't get a sense that my phones were tapped," he said.

Civil society groups from around the world condemned the US. "The UN climate talks are supposed to be about building trust – that's been under threat for years because of the US backward position on climate action – these revelations will only crack that trust further," said Meena Raman, negotiations expert from the Malaysian-based Third World Network.

"Fighting climate change is a global struggle, and these revelations clearly show that the US government is more interested in crassly protecting a few vested interests," said Brandon Wu, senior policy analyst with development organisation ActionAid in the United States.

US climate activist and founder of 350.org, Bill McKibben, called the spying revelations "insane and disgusting".

US diplomatic cables made public by WikiLeaks in 2010 showed that the CIA had sought intelligence from UN diplomats about the negotiations in advance of the summit, and Snowden documents published last year revealed the US had spied on Indonesia at the Bali climate summit in 2007.