Daniel Russel told a Senate panel the US supports the right of citizens of the semi-autonomous Chinese city to a free election for its chief executive in 2017.
Despite recent signs of warming US-China ties, the situation in Hong Kong, one of Asia's leading financial centers, sits on a long list of irritants.
Russel denied Chinese allegations that the US has helped foment the more than two months of protests, which he said weren't driven by outsiders but by Hong Kong people speaking out about their future.
On Wednesday in Hong Kong, three protest leaders surrendered to police and called for an end to the increasingly violent demonstrations.
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Students are still occupying two sites after clashes with police earlier this week as they tried to surround city government headquarters.
Russel described that flare-up as "alarming" and called for all sides to lower tensions.
Sen Marco Rubio, a possible candidate for the Republican presidential nomination, accused China of contradicting an agreement reached with Britain before the 1997 handover of the former colony.
The protesters are demanding China's government scrap its requirement that candidates in the 2017 election be approved by a panel chosen by Beijing.
Rubio said China clearly wants a Hong Kong leader that will do its own bidding and had proven itself to be an "untrustworthy partner."
Russel said China had not literally violated its commitment to allow "universal suffrage" by deciding to limit the pool of candidates, but it had fallen short of the aspirations of people in Hong Kong. A freer choice would enhance the legitimacy of the chief executive, he said.