Smoke and steam rise from a steel plant in Anyang, Henan province.

China's greenhouse gas emission targets are at risk as a result of the trade war with the United States, which has put Beijing's coal-reliant economy under heavy pressure, a senior climate official said on Friday.

"External elements, such as the Sino-U.S. trade war, have brought negative impacts and increasing uncertainties to the global economy, which has also made it more difficult for China to tackle climate change," said Li Gao, head of the climate change office at the Ministry of Ecology and Environment.

The world's biggest producer of climate-warming greenhouse gases has pledged to bring emissions to a peak by around 2030 as part of the global effort to curb rising temperatures.

It promised in June to show the "highest possible ambition" when reviewing its climate policies next year, raising hopes it would include more stringent targets in its 2021-2025 five-year plan.

However, Li, briefing reporters ahead of a United Nations climate summit in New York next month, appeared to pour cold water on the idea China would be able to significantly accelerate efforts to bring emissions to a peak.

"Don't underestimate the determination and confidence of the Chinese government, but at the same time, don't underestimate the difficulties China is facing," he said, adding, "China will not be able to meet the goal many years ahead of schedule."

China's policies are likely to come under scrutiny at the U.N. summit, especially after a summer of record-breaking temperatures across the globe.

U.N climate envoy Luis de Alba told Reuters earlier this month that the U.N. had received a "positive response" from China on ending investment in coal, a major source of greenhouse gas and air pollution.

Li said the environment ministry would focus on climate financing in the future and work with the central bank and China's top planning body to devise policies to support carbon reduction projects throughout the country, including renewables and electric vehicles.

But he said China still relied on fossil fuels and it was difficult to make adjustments, especially with the economy slowing.

"With the economy under downward pressure, the country has to take more measures to guarantee employment and the people's livelihood," he said. "Some of those measures may not fit our effort to tackle climate change."

Li also said China was still unable to meet a major requirement of the Paris climate agreement to compile a full annual carbon inventory that should be submitted to the United Nations, as the country lacked staff and resources.

China's last full inventory was for 2014, and measured the country's total emissions at 12.3 billion tonnes, up more than half in just a decade.

An influential government think tank earlier called on China to impose a 2025 cap on carbon emissions, warning that on its current trajectory, the country's annual CO2 emissions would rise 30% over the next decade.