SHANGHAI (REUTERS) - China will inject new impetus into climate change efforts after 2020, and use its Belt and Road initiative to boost cooperation in the fight on global warming, the government's top diplomat, State Councillor Wang Yi, said.

China has promised to show "the highest possible ambition" when reviewing its climate commitments next year, raising hopes that the world's biggest greenhouse gas emitter would include stronger targets in a five-year plan from 2021.

Wang, who is representing President Xi Jinping at a United Nations climate summit this week in New York, said China would aim to step up efforts to cut climate-warming greenhouse gases after next year.

A "green" Belt and Road programme would strengthen global cooperation on climate change, he added in a statement.

Beijing has already pledged a target date of around 2030 for a peak in greenhouse gas emissions, but the Paris accord on climate change urges nations to make stronger pledges if possible.

Government researchers have urged China to bring the target forward, but cautious officials have warned economic uncertainty could jeopardise even existing pledges.

"In light of tough economic and geopolitical prospects, China is weighing its options on climate," said Li Shuo, senior climate adviser with environmental group Greenpeace.

"China's statement today keeps Beijing's intentions ambiguous, leaving plenty of wiggle room for a decision to double down on climate targets in 2020."

China will also propose 150 new initiatives to mitigate annual carbon emissions by as much as 10 billion to 12 billion tonnes through the use of natural solutions, such as reforestation, Wang added.

China has championed the use of "nature-based solutions", such as the expansion of forests, grasslands and wetlands, as well as the use of biomass energy.