MOSCOW — President Vladimir V. Putin of Russia on Thursday defended his decision to fast-track Russian citizenship for some residents of separatist regions of eastern Ukraine, a move widely seen as possibly prolonging the war there and testing the mettle of the country’s novice president-elect.

“The passport question is purely a humanitarian matter,” Mr. Putin said at a news conference in Vladivostok, Russia, after a summit meeting with the North Korean leader, Kim Jong-un. Given that both the current and future Ukrainian leaders appeared intent on isolating the region, Mr. Putin said, “we cannot just look at it calmly.”

Ukraine, which does not allow dual citizenship, joined several Western nations, including France, Germany and the United States, in expressing outrage at the decision, calling it an attack on Ukrainian sovereignty.

The decision “show’s Russia’s intention to further destabilize Ukraine and to exacerbate the conflict,” the European Union said in a statement, calling on Russia to avoid steps that will impede the reintegration of the breakaway territories into Ukraine.