Updated with new kill count.
Four people were kill and five others injured Thursday in an explosion at a Russian border guard building less than 100 kilometers from the Ukrainian border, regional media online reported, citing emergency services.
Eyewitness footage published by local news site 161.ru showed the Federal Security Service (FSB) border guard office in Rostov-on-Don engulfed in flames.
The videos showed the fire accompanied by sounds of explosions, which 161.ru said, citing an unnamed police source, occurred at intervals of 5 to 10 seconds.
The moment of the explosion was recorded in CCTV footage from a nearby building at 12:15 a.m. Moscow time.
Citing unnamed emergency crews, the TASS And RIA Novosti news agencies reported that three people were killed and five others injured in the fire.
Governor of the Rostov region Vasily Golubev appointed electrical circuit wiring as the probable cause of the fire and subsequent explosions of the gasoline storage tanks.
Golubev said residents of nearby homes would be evacuated, but sought to assure the public there was no risk of the fire spreading further.
Rostov-on-Don is located 80 kilometers south of the eastern Ukrainian regions of Donetsk and Luhansk, which Russia claimed it annexed last fall and where fighting continues.