Russia is a strategic problem for the EU - Tusk

European Union foreign policy chief Federica Mogherini talks to European Council President Donald Tusk (R) during an European Union leaders summit in Brussels December 18, 2014. REUTERS/Francois Lenoir

BRUSSELS (Reuters) - Russia's approach to Ukraine and to the European Union is a challenge to the 28-nation bloc, which does not make European leaders too optimistic, but rather realistic, the new president of the European Council, Donald Tusk, said on Thursday.

"In fact Russia is today our strategic problem, not Ukraine," the former Polish prime minister told a news conference after chairing a summit at which EU leaders discussed Russia and its involvement in the conflict in eastern Ukraine.