MOSCOW -- A gunman opened fire at the headquarters of Russia's Federal Security Service in central Moscow on Thursday, killing at least one FSB agent and seriously injuring several others in a rare attack that struck at the heart of the country's feared security apparatus.

The attacker began shooting near the FSB building, known as the Lyubyanka, according to a statement from the agency, which is located around half a mile from the Kremlin in a busy shopping district. Roughly an hour after the shooting began, the agency said the gunman had been "neutralised" and there were several casualties.

At least five people were wounded in the shooting, Russian state news agencies reported, citing the country's health ministry, among them FSB agents and police officers. At least one FSB employee was reported to have been killed, according to TASS.

The FSB said it was now trying to establish the identity of the shooter, but gave few details beyond a terse statement.

Videos from the scene posted by local media showed people running as the sound of of gunshots rang out. Other videos appeared to show the outside of the FSB building being raked with automatic gunfire.

Eyewitnesses shot phone videos of heavily armed police huddled near the building and officers in helmets and body armor running toward it.

Some media reports initially suggested three attackers may have been involved. The FSB later said that was incorrect and that the gunman had been alone.

Dozens of police cars and fire engines cordoned off the building following the attack, with journalists and onlookers gathering to watch. A bomb disposal unit arrived after reports that the attacker may have been carrying explosives.

Authorities so far have not publicly commented on the attacker's motive. A spokesman for the Investigative Committee of Russia told Interfax that a case has been opened under article 317 of the criminal code, which involves an attempt on the life of law enforcement officers, but did not say whether terrorism was being considered.

Russian President Vladimir Putin had been informed about the attack, his spokesman Dmitry Peskov told Interfax.

The shooting is exceptional for Moscow, striking at a key symbol of the security services, which Putin-- a former FSB director himself-- has placed at the center of his rule. The Lyubyanka served as the headquarters of the Soviet-era KGB and a site where the secret police under Joseph Stalin carried out torture and killings.

The FSB is the main successor agency to the KGB and under Putin has become the most powerful of the country's security agencies, possessed of vast manpower and tasked with preventing terrorist attacks as well as maintaining political control in the country.

The attack also occurred the day before the FSB marks its main national holiday, the Day of the Security Services. Putin was addressing veterans of the security services at a ceremony less than a mile away in the Kremlin as the attack was unfolding. Speaking at the ceremony, Putin said the security services had successfully thwarted 33 terror plots this year.

Russia has long suffered terrorist attacks and the threat from Islamic terror groups has intensified following Moscow's military intervention in Syria in 2015. But while bombings and shooting have occurred in other cities, Moscow has not been hit for several years, a fact authorities normally attribute to the security services which they say break up dozens of plots each year.

The gunfire also erupted just hours after one of Putin's biggest events of the year-- his marathon end-of-year press conference where he takes questions from hundreds of journalists for hours at a time.

During the event this year, Putin said that two devastating terrorist attacks in the early parts of his rule had been the worst moments of his 20-year rule.