The Canadian prime minister's eyebrow became the unexpected star of the G-7 summit.
The Russian leader spoke as Trump prepares to meet with Kim Jong Un.
The summit of Group of Seven wealthy nations pitted Trump against the leaders of Germany, France, Britain, Italy, Canada and Japan on several issues.
Turkey, Saudi Arabia, the United Arab Emirates, Jordan and Qatar are to present a united front against Assad.
McMaster said the missile strike against Syria was to meant to send a "strong political message to Assad," saying the White House was "prepared to do more."
Obama is the first U.S. president to visit the city since its bombing in 1945.