The need for a more sustainable global energy system is more urgent than ever, the International Energy Agency warned on Monday as UN climate talks went into a second week.

"As international climate negotiators enter their second week of talks ... in Doha, the need to rapidly transition to a more secure, sustainable global energy system is more urgent than ever," IEA Executive Director Maria van der Hoeven said in statement.

"IEA analysis shows that achieving the internationally agreed climate goal of limiting warming to two degrees celsius is becoming more difficult and more expensive with every passing year," she said.

After six days of intense negotiations, observers in Doha said nations were far from agreement on extending the Kyoto Protocol on curbing emissions of Earth-warming gases, mainly carbon dioxide (CO2) produced from burning fossil fuels.

"In the short term, the IEA strongly encourages all governments to enact policies that promote the rapid deployment of energy-efficiency technologies; this would reduce greenhouse gas emissions and buy time to secure a much-needed global climate agreement," the statement said.

Without concerted action, the IEA forecast that announced policies "could lead to an increase (in global warning) of 3.5 degrees Celsius."

"An increase of this magnitude could trigger widespread melting of the permafrost in Arctic regions with unpredictable results," the IEA said.

Analysts say the Doha talks have become stuck, partly over a disagreement within the European Union on whether individual nations should be allowed to hold on to unused emissions quotas.

Nations could hold onto these unused allowances, estimated to amount to some 13 billion tonnes globally, under the first leg of the Kyoto Protocol that runs out on December 31.