File picture: Paul White/AP Photo.

MOSCOW - Russia's best-known campaigning newspaper asked the police on Monday to investigate after unknown individuals left nine caged sheep outside its Moscow office, some of them dressed in jackets with the word 'PRESS' and the paper's name on them.

Novaya Gazeta is well-known in Russia for its investigations into official corruption, its reporting on Chechnya, criticism of the authorities and its coverage of the opposition, in a media landscape where most newspapers are loyal to the Kremlin.

Six of its journalists, including investigative reporter Anna Politkovskaya, have been murdered since 2000, and earlier this month somebody sent a funeral wreath and a severed sheep's head to the newspaper's office with a note calling one of its reporters, Denis Korotkov, "a traitor to the motherland".

Korotkov has written extensively about the activities of Russian military contractors and Moscow's military campaign in Syria in support of Syrian President Bashar al-Assad.

Nadezhda Prusenkova, a spokeswoman for Novaya Gazeta, said security guards had spotted unknown individuals swiftly unloading the sheep from a truck on Monday afternoon before driving away.

She and her colleagues came out to find three black cages, each with three sheep inside.

Prusenkova called the incident unpleasant and said it was probably linked to earlier packages left at the paper's offices, which she said had come with threatening notes.

"We see this as a pretty unfunny joke, as someone trolling us," said Prusenkova. "We don't joke about these things. We take issues of safety (of our journalists) very seriously.

"We live in a pretty toxic environment where there are always people who are not happy with what we're doing and how we're doing it."