DURBAN, South Africa - The heads of 16 major non-governmental organizations on Thursday urged the United States to alter its stance on climate change and not to block the on- going UN climate talks in Durban.

It will clearly not be possible to reach consensus on major issues discussed in Durban if the United States insists on its "stringent preconditions," they said in a letter distributed here.

The US stance would hamper a comprehensive long-term climate regime, said the letter, addressed to US Secretary of State Hilary Clinton.

The letter maintained that the refusal by US negotiators to even allow a discussion about the relative strengths and weaknesses of climate finance proposals that had been put forward risked fostering the perception of bad faith among their counterparts.

The letter was issued as the UN climate talks entered its fourth day.

The talks hit snags after the United States rejected a proposal on how to raise $100 billion earmarked for poor countries to develop low-carbon economies and deal with the effects of global warming.

The United States reiterated its rejection at the talks, citing concerns about how the money would be raised. It wants more involvement of private businesses in the fund.

The letter also stressed the urgency of finding solutions to the world's emissions of carbon dioxide, mainly from burning fossil fuels for energy, industry and transportation.

"This is a critical meeting, and we are rapidly running out of time to avert the worst impacts of climate change," said the letter from organizations that include the Environmental Defense Fund, the Sierra Club, the Natural Resources Defense Council and other major environmental lobby and activist groups.

The letter reminded Clinton of President Barack Obama's presidential campaign pledges to move the United States back into the forefront of global cooperation on global warming.

"Three years later, America risks being viewed not as a global leader on climate change but as a major obstacle to progress," said the letter.

The U. S. refusal to discuss the Kyoto Protocol at the climate talks has dampened hope for extending the pact after its first commitment period ends by the end of next year.

The United States, the world's largest polluter per capita, has said it would not sign up for an updated Kyoto Protocol. It wants the pact to impose obligations on emerging economies like China and India.

The 1997 agreement requires 37 industrialized countries to slash carbon emissions to 5 percent below 1990 levels by 2012.