Ukraine said it was 'watching with pleasure' as footage showed a lethal fire at a building of Russia's FSB security service near border

• A fire broke out at a Russian FSB building on Thursday, causing fuel tanks to explode.
• Videos on social media show thick plumes of smoke billowing from the raging fire.

At least one person has been killed and two injured after a fire broke out at a building used by Russia's Federal Security Service (FSB) in Rostov-on-Don on Thursday, Russian news agency TASS reported.

Dramatic videos on social media show thick plumes of black smoke billowing from a raging fire.

The Governor of Rostov, Vasily Golubev, said on Telegram that a short circuit in the building's electrical wiring caused the fire, which then spread and caused fuel tanks to explode.

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He said the fire spread over 2,500 square feet and caused two walls to collapse.

While experts have determined that there is no threat of the fire spreading to neighboring apartments, residents have been evacuated as a safety precaution, Golubev said.

The FSB is Russia's primary security agency and the main successor to the Soviet Union's notorious KGB. It is responsible for internal security, counterintelligence, border security, and counter-terrorism.

The building where the fire broke out belonged to the FSB's regional border patrol section, according to the BBC.

The city of Rostov-on-Don, in southern Russia, borders the Donetsk and Luhansk regions of Ukraine, where intense fighting has been raging.

Ukraine has denied any involvement in the fire. Presidential aide Mykhaylo Podolyak said on Twitter that the fire was "a manifestation of panic" and added that "Ukraine doesn't interfere, but watches with pleasure."

There have been sporadic arson attacks on sites in Russia, such as enlistment and conscription offices, following the invasion of Ukraine last year, according to the BBC.