Two men have been detained over the killing of Boris Nemtsov, a Russian oppositioner who was shot dead late on February 27, Russian officials say.

They are named as Anzor Gubashev and Zaur Dadayev, according to the Federal Security Service (FSB) director Aleksandr Bortnikov.

Bortnikov told Russia's state-owned Pervy Kanal (First Channel) the two are "citizens of the Caucasus."

In his words, President Vladimir Putin is already aware of the detention.

According to an Interfax source, the vehicle supposedly used by the killers has also been found.

He adds that "clear pictures" of the murderers were available on the CCTV footage.

Meanwhile investigators are still working on all possible versions of the motives for Nemtsov's murfer and are so far not excluding any of them. Earlier, hypotheses of provocation, Islamic extremism, connection to the Ukraine crisis, business ties and personal hatred were all raised by the investigative body of Russia.