Russia on Friday added a Bulgarian journalist and longtime Kremlin critic to its list of "foreign agents" and ordered his arrest, in a rare use of the label against a non-Russian citizen.

Moscow has stepped up efforts to stamp out dissent after President Vladimir Putin sent troops to Ukraine in February last year, and most independent media outlets have been shut down or suspended operations.

Bulgarian Christo Grozev, 53, is the lead Russia journalist for the Bellingcat investigative website, and played a key role in its investigation into the poisoning of opposition politician Alexei Navalny.

The Russian interior ministry added Grozev to its list of foreign agents on Friday and a Moscow court ordered the arrest of Grozev, Russian news agencies reported. 

Grozev is not currently in Russia.

Moscow has extensively used the label "foreign agent", which has Stalin-era connotations, against Kremlin critics but rarely against a foreigner.

According to Russian law, it can be slapped on an individual "regardless of their citizenship." 

Late last year, Grozev was put on a wanted list by Russian authorities, and the FSB domestic security service has also accused him of helping Ukrainian intelligence.

Grozev has extensively covered Moscow's offensive in Ukraine. 

The RIA Novosti news agency on Friday quoted a source saying investigators accuse Grozev of helping prominent Russian journalist, Roman Dobrokhotov, to "illegally cross the border." 

Dobrokhotov is the founder of investigative news website The Insider.

Before the Kremlin launched its full-scale offensive last year, Bellingcat had already investigated the shooting down of flight MH17 -- which killed all 298 people in eastern Ukraine in 2014 -- and argued that Russian-backed fighters were likely responsible.

Moscow has denied any involvement in the downing of the plane.