European leaders voiced growing doubts Thursday on whether the world will meet a December deadline for a new climate deal as a summit here looked set to take up global warming in generalities.

Twenty leaders who represent 90 percent of the global economy were holding two days of talks in the eastern US city of Pittsburgh, itself billed as a model of transition from decaying steel town to a green technology hub.

The summit opened two days after a high-powered climate meet at the United Nations, where Japan and China offered new pledges on how to save the world from rising temperatures predicted to threaten entire species if unchecked.

But with just a little more than two months before a conference in Copenhagen -- designated two years ago as the venue to seal the successor to the landmark Kyoto Protocol -- pessimism was growing.

"When it comes to the negotiations, they are in fact slowing down; they are not going in the right direction," said Swedish Prime Minister Fredrik Reinfeldt, the current head of the European Union.

"We are very worried that we need to speed up the negotiations," he said.

German Chancellor Angela Merkel also sounded a sour note.

"There has been progress, in particular from the Chinese side, from the Japanese side now, and the UN meeting with (UN Secretary General) Ban Ki-moon," Merkel told reporters in Berlin before heading to Pittsburgh.

"But I have to say that when I consider what still has to be achieved before Copenhagen, we cannot be happy," she said.

World leaders are expected to discuss climate change in Pittsburgh but few expected any breakthrough. Climate negotiators, however, will meet next week in Bangkok in a new effort to make progress before Copenhagen.

The Kyoto Protocol's obligations for rich nations to cut emissions blamed for global warming expire at the end of 2012.

President Barack Obama sharply reversed the US line on global warming after assuming the White House in January by pledging to take strong action, although Congress has yet to finalize legislation on restricting emissions.

Despite their divisions over the Kyoto Protocol, developed nations are united in insisting that emerging economies must make commitments under the next treaty.

Chinese President Hu Jintao told the UN summit that the emerging power would reduce the intensity of its carbon emissions as its economy keeps growing but offered no figure.

Japan's new left-leaning prime minister, Yukio Hatoyama, pledged to ramp up the emission-cutting commitment of the world's second largest economy, in one of the few major recent gestures on climate change by developed nations.

Australian Prime Minister Kevin Rudd, who ratified the Kyoto Protocol as his first act in office in 2007, said that developed and developing nations all favored a new treaty but needed to find a "grand bargin" to seal the deal.

Rudd, speaking to students at Pittsburgh's Carnegie Mellon University, said that the Group of 20 should see a climate deal as a way toward a sustainable growth in the world economy.

"Beyond avoiding another crisis, our parallel challenge is how to generate the economic growth and the jobs of the future given that the debt-driven consumption model of the past is unlikely to be trusted anymore," he said.

A climate deal could "turn the threat of climate change into a great transformational economic opportunity," he said.