A former Russian State Duma member from the Communist Party, Denis Voronenkov, who has received Ukrainian citizenship, provided valuable evidence to the treason inquiry against Ukraine's ex-president Viktor Yanukovych, Ukraine's Prosecutor General Yuri Lutsenko said.

"We have two witnesses - an ex-deputy of the Russian State Duma, one of whom voted for the annexation of Crimea, and the other [who] did not vote. An assessment of this will also be given, but at this point his witness statements were quite important for us," he said at a briefing in Kyiv on Wednesday.

Voronenkov's testimony is very useful to the inquiry, and it is in this context that investigators are considering cooperating with him, he said.

"There are no other obligations to him [Voronenkov]," the prosecutor general said.

The treason trial against Yanukovych should begin in March, he said.

It emerged a day earlier that ex-Duma deputy Voronenkov, who left for Ukraine with his wife Maria Maksakova, also a former State Duma deputy, received Ukrainian citizenship in December 2016.