The gap has widened between countries' pledges for reducing climate-altering greenhouse gas emissions by 2020 and what is needed to keep planet warming in check, the UN warned on Wednesday.

Based on current pledges, global average temperatures could rise by three to five degrees Celsius (5.4 to 9.0 degrees Fahrenheit) this century -- way above the two degree Celsius being targeted, said a UN Environment Programme (UNEP) report.

Urgent and decisive action could still see the world get back on track, but this would mean cutting greenhouse gas emissions by 14 percent to about 44 billion tonnes in 2020 from an estimated 50.1 billion tonnes per year now.

"If no swift is taken by nations, emissions are likely to be at 58 gigatonnes (billion tonnes) in eight years' time," said a statement on the report compiled by 55 scientists from more than 20 countries.

"The opportunity for meeting the 44 Gt (gigatonne) target is narrowing annually," added UNEP executive director Achim Steiner.

Even if all countries adhered to the most ambitious level of their commitments, under the strictest rules, the gap between what has been pledged and what is needed will amount to at least 8.0 billion tonnes by 2020.

"This is 2 Gt higher than last year's assessment, with yet another year passing by," the statement said.

The agency said the concentration of warming gases like carbon dioxide (Co2) in the atmosphere had increased by 20 percent since 2000, picking up recently after a slump during the economic downturn of 2008-9.

And several countries have adjusted their non-binding reduction pledges, made under the Copenhagen Accord in 2009 -- lowering their ambition in most cases, UNEP officials told AFP.

The report, the agency's third on the topic, comes just days before the opening of UN climate talks in Doha, Qatar that will discuss an agenda for work to adopt a global pact by 2015 to enter into force by 2020.

"The sooner countries will do what they promised, the better the situation will be. But even if they do all the things they promised to do, it's still not enough if you want to stay on the path to the two degrees," UNEP expert John Christensen told AFP.