The United Nations on Wednesday hailed "concrete progress" at week-long climate talks in Bangkok, but environmental campaigners warned much faster action was needed to combat global warming.

The informal negotiations aimed to prepare for a November 26-December 7 ministerial meeting under the UN Framework Convention on Climate Change (UNFCCC) in Doha -- a stepping stone towards a worldwide emissions pact.

The UNFCCC said in a statement that the talks had made "concrete progress on key issues".

"We have a fertile ground for a successful Doha," UNFCCC Executive Secretary Christiana Figueres told reporters.

"There are still some tough political decisions ahead, but we now have a positive momentum and a greater sense of convergence that will stimulate higher-level political discussions ahead of Doha and set a faster pace of work once this year's conference begins," she added in the statement.

If approved as scheduled in 2015, the new pact would take effect in 2020, becoming the prime weapon in the fight against climate change.

For the first time, it would bring all major greenhouse-gas emitters under a single legal roof.

But who will make concessions and how the mooted treaty will work are among the many issues to be agreed, and the process will be a marathon.

Governments began preliminary discussions on the design of the accord and ways to meet the UN target to limit the rise in global average temperatures to two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.

"What I think more and more people see is that climate change is happening. We will have to do something about it," said EU chief negotiator Artur Runge-Metzger. "The sense of urgency is creeping into the negotiations."

Delegates also grappled for progress on a second commitment period for the 1997 Kyoto Protocol, whose first roster of legally-binding carbon curbs expires at the end of this year.

Agreement on the future of the Protocol -- the only treaty that binds advanced economies to targeted emissions curbs -- is key to a deal on the new treaty.

Kyoto's future lies in the hands of the European Union (EU), for it has been abandoned by almost every other major industrialised emitter.

Developing countries representing over a billion people issued a joint statement saying they feared Kyoto's environmental integrity "is eroding before our eyes" and rich countries had to beef up their emissions pledges "without conditions".

Environmentalists, meanwhile, warned that a string of recent extreme weather events around the globe including deadly typhoons, devastating floods and severe droughts show urgent action on emission cuts is needed.

They also pointed to other evidence that climate change is taking its toll, including the announcement last week that sea ice in the Arctic has shrunk to a record seasonal low this summer.

Greenpeace's Tove Maria Ryding described the climate talks as "small steps for governments, but major steps backward for humanity".

"We're seeing at the moment that ice can definitely move faster than governments," she added.

Alden Meyer, director of strategy and policy at the Union of Concerned Scientists, said the world was already seeing the "devastating impacts" of human-induced climate change.

"The time for finger-pointing, blame-casting, and hiding behind the inaction of other countries is over. We cannot afford those kind of games. What we need is political will for action and we need much greater ambition," he said.