World leaders pledged last week to step up efforts to reach a UN deal to fight climate change, but they will have to match rhetoric with rapid action to break a crippling deadlock before a December deadline.

With only two and a half months to go before 190 nations gather in Copenhagen to forge a successor to the emissions-capping pact known as the Kyoto Protocol, urgency for a breakthrough on key topics of disagreement is growing.

Progress on those outstanding roadblocks did not emerge from meetings at the UN climate change summit or at the G20 summit in Pittsburgh. Industrial and developing nations remain at odds over how to spread out greenhouse gas emission curbs. Besides, the issue of climate finance - aid from industrial countries to developing nations dealing with climate change - went largely untouched.

"The burden really falls on the US, Europe and Japan to rectify this before Copenhagen or there's not going to be the basis for a meaningful agreement," said Alden Meyer of the Union of Concerned Scientists.

Meanwhile, the fate of a US bill capping carbon emissions is expected to weigh heavily on delegates in UN climate talks that began in the Thai capital yesterday, with the Americans saying delays on the domestic front could hamper their efforts to extract concessions from other nations.

"The more specific we can be, the easier it is to press others to be equally specific," Jonathan Pershing, the chief US negotiator at the talks, said. "We have a lot of things we want from countries. ... The less we can put on the table, the harder it is to achieve that outcome."

The UN climate talks in Bangkok are drawing some 1,500 delegates from 180 countries to try to reduce a 200-page draft agreement.

In June, the House of Representatives passed the first US legislation to cap carbon emissions. The Senate is expected to take up the legislation as early as this week. But Pershing said he doubted there's enough time to pass a climate bill in Congress before the year's biggest climate change negotiations in Copenhagen in December.