US President Joe Biden and Chinese leader Xi Jinping just completed their longest exchange as world leaders - but three and a half hours of talks appear to have done little, if anything, to narrow divergent positions between the superpowers.

China's state media described the meeting as "frank, constructive, substantive and fruitful."

A senior US official said the talks, held by video conference, went on longer than expected and the two sides discussed a wide range of issues from Taiwan, to trade, to North Korea, Afghanistan and Iran.

There was nothing from the respective readouts to immediately suggest that either side had softened increasingly entrenched positions that have brought relations between the world's two largest economies to a historically volatile point, particularly over the issue of Taiwan.

And it was hard to see any definitive impact. "It appears they exchanged views about everything under the sun, but announced no decisions or policy steps," said Scott Kennedy, a China expert at Washington's Center for Strategic and International Studies think tank.

"Perhaps that will be revealed in the coming days, but if not, this ended up being a recitation of both sides’ basic positions. They seem to agree that the relationship needs to have some guardrails and stability, but they don’t agree about how to get there."

The senior US official said after the meeting that the purpose of the exchange from the US side was not particularly to ease tensions, nor necessarily was that the result.

"We were not expecting a breakthrough," the official said. "There were none to report."

Chinese media said Xi had said he hoped Biden could demonstrate "political leadership" to bring US policy towards China back to a "rational and practical" track, but appeared to offer little incentive for that, only ominous warnings.

On the key potential flashpoint of Taiwan, Xi said China would have to take decisive measures if pro-independence forces crossed a red line, while saying that the US and China were "like two ships that should not collide."

Daniel Russel, who served as the top US diplomat for Asia under former President Barack Obama and is now with the Asia Society think tank, noted it had taken 10 months for the leaders to get to the point of face-to-face talks, albeit held virtually, and suggested more could be coming.

"We should think of this not as a one-off sort-of-summit, but as one in a series of important conversations that can steer the relationship on a steadier course while the two sides continue to furiously compete," he said.

"Hopefully the Chinese side is empowering their teams to be able to hold more authoritative talks at lower levels. But this is just the beginning of process of working out of a deep hole and ultimately that requires more regular engagement between the two leaders themselves."

Paul Haenle, director of the Carnegie–Tsinghua Center for Global Policy in Beijing and a former National Security Council official, said that while the meeting has stabilized the relationship in the near term, "the long term structural challenges in the US-China relationship have not been addressed in any substantial way."

Despite the lack of obvious progress, some Chinese analysts were upbeat and Wang Huiyao, president of the Center for China and Globalization in Beijing, said that the meeting sent a "very positive signal."

"I think it will stop the downward spiral of bilateral relations and will stabilize US China relations for some time," he said, adding that it should also help reduce tensions in the Taiwan Strait.

Wu Xinbo, director of American Studies at Shanghai’s Fudan University, said the meeting continued the positive trend of improving bilateral ties following a phone call between Biden and Xi in September.

"I think both sides will turn their attention to increasing cooperation and more effective management of their differences, so as to minimize the negative impact of the frictions on bilateral ties," he said.