UN climate chief Yvo De Boer says the world is in an "all-or-nothing situation" and has urged major countries such as the United States to "act now".

"We either get a deal at the end of this week on Friday or we get nothing," De Boer, executive secretary of the United Nations Framework Convention on Climate Change (UNFCCC), said during an exclusive interview with Xinhua.

He said there were two key issues in current negotiations at the UN climate change conference here, one of which was ambitious targets and the other was ambitious financing. He hopes the US can help participating parties to "get a solution on both".

He said the climate finance offered so far was not enough and hoped "the discussion is to continue, especially on long-term finance".

De Boer asked the US to make a specific proposal on climate financing to developing countries.

US special envoy Todd Stern said earlier the U.S. did not owe climate debt to developing countries and would not offer any financial help to China on its mitigation and adaption efforts.

"The US has a problem with the concept of historical responsibility," De Boer said, stressing "the UNFCCC states very clearly that actions by developing countries (to fight climate change) is conditional on support by rich countries, we need to respect that here."

On the current US position on climate change, De Boer said the country was in a "very difficult situation", since most countries started to act on climate change under the Kyoto Protocol years ago while the U.S. did not.

Most countries take 1990 as a base year, while the US doesn't, and the U.S. carbon dioxide emissions are already 14 percent above the 1990 level.

The reduction US President Barack Obama had proposed was significant, said De Boer, "the question is whether there is something that the US could do more."

He said it was good President Obama was coming to this conference and hoped the U.S. president could help find a solution.

He said the whole world would pressure the US to do more, but it was hard to predict whether that pressure was enough.

De Boer said China's commitment to cut carbon dioxide emissions per unit of gross domestic product (GDP) by 40 to 45 percent by 2020 on the basis of 2005 was "very encouraging".

"I think that is very significant," he said, "This is building on a five-year plan that is already in place and I find it encouraging."

The world needed to cut CO2 emissions by at least 25 percent by2020 from the 1990 level to avoid the two-degree increase above pre-industrial levels and to head off the worst of global warming, the U.N. climate chief stressed.

He said the current negotiations were "very, very difficult", stressing "we need to see an advance, especially on the Kyoto Protocol."

"Once we have that, we can move smoothly," he said, adding "we need to see targets from the Kyoto Protocol parties in the next commitment period. Once we have that, the rest will move smoothly."