One person was killed and two more were injured in southern Russia Thursday when a fire tore through FSB security service offices in a region bordering Ukraine.
Kyiv denied direct involvement in the incident but said authorities were following the events "with pleasure".
Images circulating on social media showed a massive blaze tearing through the office block in a built-up urban area with a huge plume of black smoke billowing over the fire.
Several nearby buildings in the city of Rostov, some 120 kilometres (75 miles from the Ukraine border), were evacuated as a precaution, the regional governor announced.
The official, Vasily Golubev, said the fire spread over 800 square metres (8,600 square feet) and was sparked by a "short circuit", causing "the explosion of fuel and lubricant containers".
There have been several incidents of reported sabotage attributed to Ukrainian partisans within Russian territory since the Kremlin deployed troops to Ukraine in February last year.
A senior Ukrainian presidential aide Mykhaylo Podolyak said the incident instead pointed to "panic" and "internal conflict."
"Ukraine doesn't interfere, but watches with pleasure..." he added in a statement on Twitter.
