BERLIN -- Senior French and German officials have warned that the surge in nationalism fueling the crisis in Ukraine echoes the ideas that gripped Europe a century ago and led to the First World War.

Sigmar Gabriel, Germany's vice chancellor, says the "ghosts of nationalism are being let out of the bottle" in Ukraine, where separatists in the East are seeking to join Russia.

Gabriel said Monday the crisis in Ukraine demands "more, not less diplomacy" from Europe, and added that "saber-rattling by NATO" wouldn't solve the problem.

French Prime Minister Manuel Valls called the Ukraine crisis "probably the greatest risk to peace and stability in Europe since the Iron Curtain came down."

Gabriel and Valls spoke during an event commemorating the First World War at Berlin's French Cathedral, a symbol of Franco-German friendship.