Authorities in Ukraine say the sole witness in the killing in Moscow of prominent Russian opposition figure Boris Nemtsov has received death threats since returning to her family home in Ukraine.

The General Prosecutor's office in Kiev said in a statement Friday that Anna Duritskaya, the 23-year old model who was walking with Nemtsov at the time of his killing, is being provided with protection. The statement offered no information on who might have issued the threats.

Duritskaya was detained by Russian police for several days of questioning before she was permitted to leave the country.

Russian President Vladimir Putin has called the slaying of his top critic a "disgrace" to Russia, although scant progress has been made in the investigation into the shooting last Friday night.