Pro-Ukrainian saboteurs were involved in the recent spate of explosions at Russian military sites in Crimea, a Ukrainian government official told NBC News.

The series of blasts hit military depots and airbases in the annexed peninsula over the past week, hinting at a growing ability by Ukraine’s military or its backers to strike deep behind enemy lines, a development that could shift the dynamics of the war…

The official declined to say whether the Ukrainian military or special forces were also involved in the attacks. But he added, “Only thanks to the people who oppose Putin in the occupied territories and in Russia today, resistance is possible.”

Explosions were reported near an airbase Thursday night in Crimea, on the southwest coast near the port city of Sevastopol, the headquarters of Russia’s Black Sea Fleet. On the opposite end of the peninsula, the sky was also lit up at Kerch near a huge bridge to Russia, in what Russia said was fire from its air defenses.