New Delhi, June 4 -- A massive black cloud rising above the St. Petersburg skyline from a Ukrainian drone strike set a gloomy tone for the opening of President Vladimir Putin's annual showcase of Russia's economic achievements.

With Putin set to arrive Thursday in his hometown that is hosting the St. Petersburg International Economic Forum, the Ukrainian attack a day earlier that set an oil terminal ablaze was another embarrassing blow to his efforts to minimize the impact of the 4-year-old conflict and cast it as a distant event with no effect on Russian daily life.

The attack, which also targeted a naval base near Russia's second-largest city on the Gulf of Finland, underlined Ukraine's growing capability to hit deep inside its neighb...