US Energy Secretary Steven Chu says the US will not be able to cut greenhouse emissions as much as it should due to domestic political opposition.

Prof Chu told BBC News he feared the world might be heading towards a tipping point on climate change.

This meant the US had to cut emissions urgently - even if compromises were needed to get new laws approved.

Environmentalists said Prof Chu, a Nobel physicist, should be guided by science not politics.

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"With each successive year the news on climate change has not been good and there's a growing sensation that the world and the US in particular has to get moving," he said.

"As someone very concerned about climate I want to be as aggressive as possible but I also want to get started. And if we say we want something much more aggressive on the early timescales that would draw considerable opposition and that would delay the process for several years.

The US energy secretary said that awareness of climate tipping points had increased greatly only in the past five years. He added: "But if I am going to say we need to do much, much better I am afraid the US won't get started."

To the anger of environmentalists, he said that one compromise would be approving new coal-fired power plants without obliging them to capture and store their carbon. The UK government has made this a stipulation for new coal plants but Prof Chu declined to explain why the US government would not follow suit.

Maybe if the Democrats decided to engage the Repuglicans, instead of cowering away as usual, these important environmental measures could be made into law.  That requires a backbone though, and we all know Harry Reid can not seem to find his lately, if ever.