The elusive commander of the pro-Russia militants who have seized the east Ukrainian city of Slovyansk has revealed himself for the first time since the crisis began, saying in a taped interview that his armed crew arrived in Ukraine's east from Crimea.

Igor Strelkov, the commander whom officials in Kiev have described as a Russian intelligence officer, presented some details of the fighters he brought to Slovyansk, who since early April have transformed the city into the epicenter of eastern Ukraine's unrest. The new government...