Boxing legend Muhammad Ali welcomed the Prince of Wales and Duchess of Cornwall to his home city of Louisville, describing Charles as "the greatest".
The sport star's words came in a letter rather than in person as he was not well enough to join leading figures from the city at a cultural greeting for the royal couple, who were in the city on the final day of their US tour.
Prince Charles has said there is "awful lot to worry about" in relation to the world's environmental resources and wildlife.
He said his conservation efforts were driven by a need to leave future generations a world "which isn't even more destroyed and damaged and dysfunctional than it need be".
He listed his areas of concern from the continuing destruction of rainforests, to the threat to endangered animals like rhinos, elephants and tigers and the need for sustainable cities.
Charles said: "The world has looked to the United States for leadership in so many challenging circumstances in the past.
"However, today we are faced by truly exceptional challenges and threats - a veritable 'perfect storm' which, if not met by strong, decisive and far-sighted leadership, could overwhelm our capacity to rectify the damage and thereby destroy our grandchildren's future inheritance."
His comments came after he was presented with the exceptional leadership in conservation honour, from the International Conservation Caucus Foundation during a ceremony in Washington.
Barack Obama told Prince Charles that American people rate UK royalty above their own politicians on the royal couple's Washington tour.
Watch Prince Charles try - and fail - to get a strike on the bowling alley during a trip to a veterans' retirement home in Washington.
Charles and Camilla spent a busy first full day of their US trip visiting memorials to President Abraham Lincoln and Martin Luther King Jnr.