US Treasury Secretary Janet Yellen has called on China to work with Washington to fight the "existential threat" of climate change. Speaking on Saturday, she said the two countries — the largest greenhouse gas emitters — had a joint responsibility to lead the way on climate action. She called on China to support the US-led Green Climate Fund. Yellen is on a four-day trip to Beijing in an attempt to boost relations between the two countries. Chinese Vice Premier He Lifeng, who was among those to meet with Yellen, said he regretted "unexpected incidents", such as the row over a spy balloon, had hurt ties with the United States. There's been no formal cooperation between China and the US on climate change since the administration of former President Donald Trump. And China briefly suspended climate talks entirely with the US last year after senior Democrat Nancy Pelosi visited Taiwan, which is self-ruled but Beijing sees as a breakaway province it will eventually unite with. But in a sign that cooperation could soon resume, Yellen called on China to work together with the US to fight climate change and mitigate the effects on poorer countries. During the roundtable meeting in Beijing with finance experts, she called on China to support US-led institutions like the Green Climate Fund, which was set up to help developing nations adapt to climate change and lessen its effects. "As the world's two largest emitters of greenhouse gases and the largest investors in renewable energy, we have both a joint responsibility — and ability — to lead the way," she said. China is now the world's biggest investor in solar energy, and biggest producer of solar panels and wind turbines but saw its carbon dioxide emissions rise 4% in the first quarter of this year compared to 2022. The US, meanwhile, has invested billions of dollars in recent years into initiatives aimed at tackling climate change but also saw its emissions rise slightly last year, according to the International Energy Agency. While Yellen wants China to join the US in funding the worldwide transition to renewables, the sticking point is China's insistence that it is still a developing country. Beijing says it is up to the US and Europe to pay for the energy transition, because they have historically created most of the emissions. Yellen is the second senior Washington official to visit Beijing in the last two months. Her presence there is aimed at easing tensions and restoring ties between the world's two superpowers. US Secretary of State Antony Blinken visited Beijing last month, making him the highest-ranking Washington official to visit the Chinese capital in almost half a decade. He met President Xi Jinping and Foreign Minister Qin Gang. At the end of his trip, Blinken said that while there were still major issues between the two countries, he hoped they would have "better communications, better engagement going forward." However, the next day President Joe Biden referred to Xi as a "dictator" — triggering outrage from Beijing. In another sign the trade dispute between the two countries is far from being resolved, China this week announced it was tightening controls over exports of two materials crucial to producing computer chips. From next month, special licenses will be needed to export gallium and germanium from China, which is the world's biggest producer of the metals. — BBC