A top US official lashed out Monday at China's "confounding" failure to "step up" on global action to manage the climate crisis.

President Joe Biden's national security advisor, Jake Sullivan, said most of the world would mark progress at the COP26 climate summit in Glasgow, thanks to a "real jolt of energy" over the course of 2021.

"Now the question is can we get the world aligned?" Sullivan asked aboard Air Force One, which was due to land shortly in Scotland.

"There are significant outliers," he told reporters, "one of those significant outliers being China, who will not be represented at leader level at COP26 and who we do believe has an obligation to step up to greater ambition as we go forward."

"We'll keep pressing on that," he said, calling the contrast between US efforts under Biden to fight climate change and what he said was China's passive stance "confounding."

President Xi Jinping is one of the notable missing names at the big UN conference, which runs for two weeks from Monday.