Beijing and Hong Kong authorities should "work together to ensure a competitive process for the election of the chief executive," the top US diplomat for Asia, Daniel Russel, told US lawmakers.
He added that Washington saw its role as fostering such elections, while speaking up in support of the protesters, adding the US takes "the side of justice, we take the side of freedom."
Russel was speaking as the founders of the pro-democracy movement surrendered to police in a symbolic move, seeking to take the protests off the streets after more than two months of rallies which have seen sporadic violence.
Protesters began blocking three major Hong Kong intersections in late September to demand free leadership elections in the semi-autonomous Chinese city which was handed back to China from British rule in 1997.
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But China insists that candidates for the 2017 leadership vote must be vetted by a loyalist committee, which demonstrators say will ensure the election of a pro-Beijing stooge.
"The legitimacy of Hong Kong's chief executive will be greatly enhanced if the promise of universal suffrage is fulfilled," Russel told the Senate Foreign Relations committee.
"This means allowing for a competitive election in which a range of candidates with differing policy approaches are given an opportunity to seek the support of eligible Hong Kong voters," he insisted.