Green groups reacted with concern at the world climate talks on Wednesday after Japan stood by its opposition to renewing pledges under the Kyoto Protocol, the cornerstone UN pact on global warming.

Friends of the Earth said Japan had thrown down an obstacle at Cancun, where the future of the Protocol is part of a complex, interlinked haggle, by speaking so bluntly.

In a plenary session at the start of the talks on Monday, Japan said it did not plan to renew its carbon-cutting pledges under Kyoto when the current roster of promises expire at the end of 2012, delegates said.

In an argument it has repeated over nearly a year, Japan says Kyoto's targeted carbon constraints are unfair and ineffective in the present arrangements for tackling global warming.

They only apply to rich countries, but not the United States, which abandoned the treaty in 2001, nor to China, the world's No. 1 polluter, which is a developing country. As a result, only 30 percent of planet-wide emissions of greenhouse gases are covered.

"With this position, Japan isolates itself from the rest of the world. Even worse, this step undermines the ongoing talks and is a serious threat to the progress needed here in Cancun," said Yuri Onodera of Friends of the Earth Japan.

The UN talks in Cancun are essentially two tracks, with linkages.

One track gathers all 194 parties under the UN Framework Convention on Climate Change (UNFCCC), including the United States, on worldwide action beyond 2012.

The other gathers only the Kyoto Protocol parties. Developing countries have said in essence they will commit to a deal in the first track provided there is a deal in the second track for renewing the Protocol.

As a result, if Japan and other rich countries bale out of Kyoto, that could block a deal in the worldwide arena.

"It's clear we're strongly disappointed," Greenpeace's Wendel Trio told AFP.

"At the same time, we do see that the EU is really committed to continuing a second period and that position is not dependent on Japan. I still think there is a number of countries, including Australia, New Zealand and Russia, that want to continue."

UN climate chief Christiana Figueres said Japan's statement was "not new" and underscored the long and complex process of negotiations.

"The position of Japan in respect with the KP has been on the table for 11 months. It should not take anybody by surprise," she told reporters.

"It is clear that parties are going to have to compromise, that they are going to have to flexible, that they are going to have to understand each other's position and situation, and try to find where the median ground may lie," Figueres said.