MOSCOW - The chief witness to the murder of Russian protest leader Boris Nemtsov, a Ukrainian model by his side when he was shot dead, complained Monday of being kept under guard in Moscow.

Ganna Duritska, 23, said she had given all the information she could to investigators but that they were preventing her from leaving Russia "for her security". "For three days they have escorted me in police cars to the Investigative Committee," Duritska told the Dozhd television channel, referring to the agency in charge of the probe.

"They don't explain when I will be let go or for what reason I am kept here." Nemtsov, 55, was gunned down close to midnight Friday in a heavily policed area on a bridge just metres from the Kremlin, with the assailants still at large despite a citywide manhunt.

Speaking via a fuzzy Skype connection from a Moscow apartment, Duritska said she did not see where the assassin came from as the attack took place behind her.

But she said she noticed a light-coloured car quickly drive off. She said there had been no earlier indication of any danger, or any sign of surveillance, adding that the first police car arrived 10 minutes after she had called. She said she was immediately taken in for questioning which lasted through the night, with data extracted from her phones by the investigators, adding that she is being questioned as a witness.

"I've done everything I could," she said, but investigators have told her that it would be unsafe to leave Moscow. "They say it's for my security." "I don't understand why I am still on Russian territory, because I want to go to my mother who is ill and is in a difficult psychological state." Ganna's mother Inna Duritska, who lives in Kiev, told AFP that her daughter's virtual house arrest made her fear foul play on the part of investigators.

"I am afraid that she will be accused of this murder because they need the Ukrainian trace. They can invent anything they want," she said. Moscow and Kiev have been arch foes since regime change in Ukraine last year which installed a pro-Western government that has been battling a pro-Russian insurgency in the east since April. "I'm afraid that my daughter will become a second Nadezhda Savchenko," Inna Duritska said, referring to the Ukrainian pilot who is on hunger strike in custody in Moscow, accused of involvement in the murder of Russian citizens during the Ukraine conflict.

"She is under huge psychological pressure. They checked her telephone and are threatening to take her to a polygraph in handcuffs," she added, saying her daughter called her in tears. "They have no right to treat her that way!" Ukrainian foreign ministry spokesman Yevhen Perebyinis said the consul in Russia had been to see Duritska several times and that she has written a formal letter requesting her return home. "The Ukrainian embassy has issued a note asking to ensure the lawful return of Duritska to Ukraine," he told AFP.