Tusk spoke at a news conference in Warsaw as Poland took over the rotating presidency of the EU, the EUobserver reported. He said the EU is threatened by a "new euroscepticism."
"The union is going through one of the most difficult and complicated moments in its history," he said.
Traditional eurosceptics were ideologically opposed to the idea of a united Europe, Tusk said. New ones "say they support the EU but at the same time take steps that weaken the union."
Tusk did not name specific leaders. But he pointed to France and Italy putting up border controls and Germany's reluctance to back a second bailout for Greece.
"People are saying to one another: 'You're inadequate. You should leave the EU. You should leave the euro.' I see such tendencies and I believe the right path is the opposite one. ... I am convinced that what the EU needs is further integration," he said.
Notable deaths of 2014 [PHOTOS]