By Valerie Volcovici

BEIJING (Reuters) - Climate change is a "universal threat" that should be handled separately from broader diplomatic issues, U.S. climate envoy John Kerry told Chinese Vice-President Han Zheng on Wednesday after two days of what he called constructive but complex talks.

Acknowledging the diplomatic difficulties between the two sides in recent years, Kerry said climate should be treated as a "free-standing" challenge that requires the collective efforts of the world's largest economies to resolve.

"We have the ability to ... make a difference with respect to climate," he said at a meeting at Beijing's Great Hall of the People, China's sprawling parliament building.

Kerry arrived in Beijing on Sunday as heat waves scorched parts of Europe, Asia and the United States, underscoring the need for governments to take drastic action to reduce carbon emissions, which contribute to global warming and extreme weather events.

He has held meetings with China's top diplomat Wang Yi and Premier Li Qiang as well as veteran climate envoy Xie Zhenhua in a bid to rebuild trust between the two sides ahead of COP28 climate talks in Dubai at the end of the year.

"If we can come together over these next months leading up to COP28, which will be the most important since Paris, we will have an opportunity to be able to make a profound difference on this issue," he told Han.

Han said the two countries had maintained close communication and dialogue on climate since Kerry's appointment as envoy, adding that a joint statement issued by the two sides has sent a "positive signal" to the world.

Kerry told reporters earlier that his talks with Chinese officials this week have been constructive but complicated, with the two sides still dealing with political "externalities", including Taiwan.

"We're just reconnecting," he said. "We're trying to re-establish the process we have worked on for years."

"We're trying to carve out a very clear path to the COP to be able to cooperate and work as we have wanted to with all the externalities," Kerry said.

Climate diplomacy between the world's top two emitters was suspended in August last year following the visit of U.S. House of Representatives Speaker Nancy Pelosi to Taiwan, a democratically governed island that China claims.

"The mood is very, very positive," Kerry said ahead of Wednesday's meetings. "We had a terrific dinner last night. We had a lot of back and forth. It's really constructive."

"We're focused on the substance of what we can really work on and what we can make happen."