PARIS , France - The joint China-US climate declaration is "a significant moment" in the build-up to COP28, the climate summit's president said Wednesday, but some experts criticised the lack of concrete commitments.

"The consensus reached between the US and China is a significant moment ahead of COP28" said Sultan Al Jaber, president of the 28th UN climate conference set to take place in Dubai at the end of the month.

"It clearly signals that despite global challenges, COP28's call for climate action is uniting parties and raising ambition," he added in a statement.

China and the United States pledged Wednesday to work more closely together to fight global warming, declaring the climate crisis "one of the greatest challenges of our time".

The announcement came hours ahead of the first meeting between Presidents Joe Biden and Xi Jinping in a year, on the side-lines of the Asia-Pacific Economic Cooperation (APEC) summit in San Francisco.

Petteri Taalas, head of the United Nations' climate and weather agency, welcomed the announcement.

"It's important that those biggest emitters -- China and the USA -- are supporting further ambition level of climate mitigation," the World Meteorological Organization chief said.

"The forthcoming COP, we will see what happens there, but this fact that the USA and China have started interacting is a positive sign because those big players, they can change the big scene," he told a Geneva press conference.

"Let's hope that this enhanced potential of US-China cooperation will lead to concrete results."

'No major breakthrough'

Li Shuo, a former Greenpeace staffer now at the Asia Society Policy Institute, said the declaration was "good news" and "it sets a floor for the meeting in the UAE".

"That said, this statement did not see any major breakthrough in terms of what the two countries committed to do.

"There will still be a lot of contentious issues" to discuss at COP28, he added.

David Waskow, of US think-tank the World Resources Institute, called it "disappointing that the two nations said nothing about the need to rapidly transition away from fossil fuels this decade, which will be a central issue at the COP28 summit".

"The most striking part of the statement is the two countries' commitment to include all greenhouse gas emissions, including methane, in their next national climate plans", he added.

Waskow called the inclusion of methane "a major step" as China is the world's largest methane emitter.

Methane is the second greenhouse gas of anthropogenic origin (linked to human activity) after carbon dioxide. But its warming effect is 28 times greater than that of carbon dioxide over a 100-year horizon.