A lawyer for one of two Russian servicemen jailed in Ukraine says the groundwork is being laid for a possible exchange with Ukrainian pilot Nadezhda Savchenko, who is imprisoned in Russia.

Lawyer Valentin Rybin told the state news agency Tass on Wednesday that both Russians have submitted a petition for a pardon to Ukrainian President Petro Poroshenko.

He said the same procedure is underway in Russia regarding Savchenko and the outcome will be clear by evening.

One of Savchenko's lawyers, Mark Feygin, said Tuesday that he was trying to persuade Savchenko to file a petition for pardon.

Savchenko was captured by Russia-backed rebels in eastern Ukraine in 2014 and sentenced in March to 22 years in prison for her alleged role in the deaths of two Russian journalists.