GLASGOW, United Kingdom -- US President Joe Biden on Tuesday accused China and Russia of failing to show leadership on climate change in blistering criticism of their leaders for not attending the COP26 summit in Glasgow.

Speaking at the UN summit aimed at forging an ambitious new climate agreement, Biden called his own presence and promises proof that "America is back" after the go-it-alone approach of his predecessor Donald Trump.

"The fact that China is trying to assert, understandably, a new role in the world as a world leader -- not showing up, come on!" Biden told journalists before flying out of Glasgow.

"It just is a gigantic issue and they walked away. How do you do that and claim to be able to have any leadership?" Biden said.

"It's been a big mistake, quite frankly, for China not showing up. The rest of the world looked at China and said 'what value are they providing?'," he added.

Xi, who leads the world's largest emitter of carbon emissions responsible for climate change, has not travelled outside of China since the beginnings of the Covid-19 pandemic in early 2020.

Biden was even more scathing about Russian President Vladimir Putin, who does travel and met the US president in Geneva in June. Russia is the world's fourth largest emitter.

"His tundra is burning -- literally, the tundra is burning. He has serious, serious climate problems, and he is mum on willingness to do anything," he said.

- America is back -

Biden has ramped up US climate action with promises to zero out carbon emissions by 2050, a sharp reversal from the climate-skeptic Trump, although Biden still faces domestic hurdles on moving ahead.

Biden said he was making good on his vow on his first international trip as president -- to the Group of Seven summit in Cornwall in June -- that the United States was returning to the international stage.

"Two world leaders came up to me today and said, 'Thank you for your leadership. You're making a big difference here,'" Biden said, acknowledging his comments sounded "self-serving."

US officials had earlier expected Xi to meet Biden for the first time as president at this weekend's Group of 20 summit in Rome.

The two countries, however, instead said they would meet virtually by the end of the year. Biden said no date has been set.

Biden said he hoped their talks would bring more predictability in relations that have been soured by disputes on myriad fronts, including human rights and China's growing assertiveness on Taiwan.

"I'm going to be clear. This is competition; it does not have to be conflict," Biden said.

"I have also indicated to him -- and I'm not reluctant to say it publicly -- that we expect him to play by the rules of the road."

Biden recently made waves by appearing to say the United States would militarily defend Taiwan if it was attacked by China, which claims the self-governing democracy.

The United States provides weapons to the island but is deliberately ambiguous on whether it would defend it.

Without addressing Taiwan directly, Biden said he did not believe there would be conflict with China, which his administration has identified as the top challenge of the 21st century.

"I don't anticipate there will be a need for physical conflict," Biden said.