Two men attacked another man near a London military barracks on Wednesday. One man died and the two others believed to be have weapons were shot by police.
Speaking at a press conference in Paris with French President Francois Hollande, Cameron called the attack "the most appalling crime" and said there are "strong indications" that it is a terrorist incident.
Hollande says a British soldier was killed in the attack, but Cameron didn't immediately confirm that.
Cameron said he would be cutting his trip short to return to London.