MOSCOW: Captured on a CCTV camera late at night, a man on a bicycle with a food delivery backpack approaches the offices of Novaya Gazeta, Russia's leading independent newspaper.

In the grainy footage recorded in central Moscow earlier this month, the man sprays an unknown substance at the newspaper's front door, then slowly walks away, careful not to look up at the camera.

Sitting in the offices a few days later, editor-in-chief Dmitry Muratov tells AFP he has no doubt the "chemical attack" was the latest attempt to silence one of the few media outlets in Russia willing to challenge the official line.

"This is the use of military-grade non-lethal poisonous substances to warn the paper's staff or as revenge on them," says Muratov, a grey-bearded 59-year-old who since 1995 has served several times as Novaya Gazeta's editor-in-chief.

Several employees felt unwell after the attack and it took days of cleaning to get rid of the horrible stench, with the strip of pavement that was sprayed having to be removed.

It was hardly the first attack on Novaya Gazeta, and far from the worst.

Since the early 2000s, six Novaya Gazeta journalists have been killed in connection with their work -- their black-and-white portraits now hang together in the newspaper's office.

"It's no secret that when they killed Anna Politkovskaya I wanted to close this newspaper... This newspaper is dangerous for people's lives," Muratov says.

Politkovskaya, who spent years reporting for Novaya Gazeta on rights abuses and the Kremlin's war in the North Caucasus region of Chechnya, was shot dead outside her apartment in 2006 at the age of 48.

"The newspaper's journalists were categorically against this, they believed we would violate the memory of Anna Politkovskaya if we closed down... They convinced me," Muratov says.

Muratov was among a group of journalists who founded Novaya Gazeta in 1993, inspired by the newfound freedoms that followed the collapse of the Soviet Union.