Ukrainian police have opened a criminal investigation after Hanna Durytska, a key witness in the murder of Russian opposition politician Boris Nemtsov on February 27, received threats, the Ukrainian Prosecutor General's Office said in a press release on Friday morning.

"Prosecutor General of Ukraine Viktor Shokin has ordered that all necessary measures be taken to protect the life and health of Hanna Durytska, a witness in the killing of Russian opposition politician Boris Nemtsov," it said.

On March 5, Durytska alerted the Bila Tserkva police department in Kyiv region that she had received death threats from unknown people while staying with her parents, the Prosecutor General's Office said.

Acting on orders issued by the Kyiv region Prosecutor's Office, the Bila Tserkva police department opened a criminal investigation on counts of "attempted premeditated murder", it said.

All measures are now being taken in order to protect Durytska's life and health. The Ukrainian prosecutor general is personally coordinating the inquiry.

Nemtsov was fatally shot in central Moscow at around 11.30 p.m. on February 27, as he was walking with Durytska along the Bolshoy Moskvoretsky Bridge. Russian investigators are probing several lines of inquiry, including political and personal motives.

On March 2, Durytska returned to Kyiv accompanied by her lawyer.

Russian Investigative Committee spokesman Vladimir Markin said that Durytska had signed a gagging order and had agreed to further cooperate with Russian investigators.