Russia's Federal Security Service (FSB) says one of its members was killed and five were wounded by an unidentified shooter who opened fire outside its headquarters in central Moscow on December 19.



"One FSB officer was killed in a shootout in central Moscow," the security agency told Russian media, saying that the assailant was acting alone and didn't enter the building. Earlier, the FSB said the assailant had been “neutralized” -- a term usually used by Russian officials when an attacker is killed.



"An unknown individual opened fire near building number 12 on Bolshaya Lubyanka street, there are casualties. The identity of the criminal is being established. The criminal has been neutralized," the FSB told the media.



The FSB is the main successor agency to the Soviet-era KGB.



A spokesman for Russia's Health Ministry was quoted as saying that the five people wounded in the shooting were receiving medical care and that two FSB officers "received very serious wounds."



Crowds could be seen in video posted online fleeing the central Lubyanka Square amid the sound of gunfire.



Kremlin spokesman Dmitry Peskov said President Vladimir Putin was informed about the incident that came shortly after the Russian leader's annual news conference.



The shooting also occurred on the eve of the Day of Russia's Security Services, celebrated on December 20.



Roughly around the same time when the incident occurred, Putin, a former KGB officer, delivered a speech at a Kremlin concert marking the security services' day. He said that at least 54-terror related crimes had been thwarted in Russia in 2019, including 33 attacks.



The FSB denied earlier reports that the attack had been perpetrated by three assailants.



"The perpetrator acted alone," the FSB said.



The Kremlin-funded RT television channel reported on social media that Russian intelligence services had described the shooting as "an act of terrorism."



Hours after the incident, the area around the FSB building remained blocked and public transport was rerouted.

Numerous law enforcement officers were still present at the scene.

With reporting by Reuters, AP, AFP, dpa, Interfax, and TASS