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US President has urged Americans to show unity, "our greatest strength," in a video posted Friday, on the eve of the .

"To me, that's the central lesson of September 11th. It's that at our most vulnerable, in the push and pull of all that makes us human, in the battle for the soul of America, unity is our greatest strength," Biden said in a six-minute message from the .

On Saturday he and First Lady Jill Biden : New York, the Pentagon, and a field in Pennsylvania.

But Biden, much criticized for his management of the US withdrawal from and struggling to contain the Covid-19 pandemic, is not expected to speak in public during the ceremonies.

"Unity doesn't mean we have to believe the same thing, but we must have a fundamental respect and faith in each other and in this nation," Biden said in his video remarks.