Vyacheslav Gladkov, the governor of Belgorod region, said in a statement that emergency services were investigating the cause of the fire near the settlements of Timonovo and Soloti, 15km from Ukraine. He did not give details.

A popular Ukrainian news website posted a video it said showed a large ball of flame erupting from the base. Reuters was unable to immediately verify the video was genuine.

Russia on Tuesday blamed saboteurs for orchestrating a series of explosions at an ammunition depot in Russian-annexed Crimea, further to the south. Last week, blasts ripped through a Crimean air base, which Moscow at the time said was caused by an accident. Read full story

After months in which Russian forces made modest territorial gains in heavy fighting in the east since its February 24 invasion of Ukraine, front lines have been comparatively static for weeks. The government in Kyiv has said it was preparing for a counteroffensive to recapture a swathe of southern territory. Read full story

Gladkov has in previous months blamed Ukrainian forces for a series of blasts in and around Belgorod, which is about 650km south of Moscow.

(Reporting by David Ljunggren; Editing by Grant McCool.)