The German Chancellor was unimpressed with Trump's failure to commit to the Paris climate deal during the G7 summit.
The summit of Group of Seven wealthy nations pitted Trump against the leaders of Germany, France, Britain, Italy, Canada and Japan on several issues.
Mount Etna erupted 22 times already this month.
The president's unpredictability is worrying America's friends abroad, but there are signs that his hardline, "America First" isolationism is softening.
Obama is the first U.S. president to visit the city since its bombing in 1945.
EC president's top aide also said a Johnson premiership would be a "horror scenario."
French finance minister says a "Brexit" would roil financial markets.