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.

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 degrees 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.

Without swift action, emissions are likely to be at 58 gigatonnes (billion tonnes) in eight years' time, according to the report compiled by 55 scientists from more than 20 countries.

"The opportunity for meeting the 44 Gt (gigatonne) target is narrowing annually," said UNEP executive director Achim Steiner -- adding the message was "one of great alarm and concern about where we are".

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 gigatonnes higher than last year's assessment, with yet another year passing by," said a statement.

The so-called emissions gap could soar to 13 gigatonnes in the worst case scenario in which countries take a lenient approach to their emissions commitments, said the UNEP.

Based on countries' current actions, it was "more probable than not that the gap in 2020 will be at the high end," of the range, it warned.

"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.

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

And several countries have lowered their non-binding reduction pledges made under the Copenhagen Accord in 2009, UNEP officials told AFP.

Even if the 2020 targets are achieved, countries would have to make steep cuts to keep warming at two degree Celsius after that, said the report, with emission levels dropping at a median 2.5 percent per year to 37 gigatonnes by 2030 and 21 gigatonnes by 2050.

The report 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.

Achim stressed the bad news "is not a reason to write off the two-degree target that the world has set itself."

Large emissions cuts are possible, to the tune of some 17 gigatonnes, from greening sectors such as building, power generation and transport, according to the report.

"The technical potential does exist to bring emissions down by 2020 and close the gap," UNEP chief scientist Joseph Alcamo told a teleconference.