Ukrainian President Petro Poroshenko says he has agreed "a certain model" for the release of  Nadiya Savchenko, a pilot of his air force recently jailed in Russia for 22 years for the killing in 2014 of two Russian journalists in the Donbas conflict.

"Yesterday I initiated a [phone] conversation with [President Vladimir] Putin and based on preliminary preparations it seems to me we have managed to agree on a certain algorithm to free Nadezhda," Reuters quoted Poroshenko as saying at a press conference with Danish Prime Minister Lars Rasmussen in Kyiv on April 19.

Poroshenko added that a court verdict on two Russian servicemen captured in the Donbas, Yevgeny Yerofeyev and Alexander Alexandrov, who were sentenced by a Ukrainian court to 14 years in jail for terrorism on April 18, "suggests certain possibilities for employing a mechanism of exchange".

The Kremlin refused to comment on a possible prisoner exchange, but Poroshenko's statements indicate the sides will agree to a straight swap of the prisoners that analysts say can play well for both leaders at home.

"For Putin, it is one less source of stress with the West, and he can play the line that he is being reasonable," Nomura International analyst Tim Ash wrote. "While domestically it plays well to the domestic audience, securing the release of a couple of captive Russian soldiers in the run- up to Duma elections [in September is beneficial]. Poroshenko meanwhile can send a message that under his new [Volodymyr] Groysman government things get done, he is resolving outstanding issues, and also moving the debate on from the Panama papers."

While Russia denies deploying troops in East Ukraine, special forces officers Alexandrov and Yerofeyev were taken prisoner there last May. Savchenko was captured by separatist units 11 months earlier and found guilty of directing Ukrainian artillery fire onto two journalists of Russia's VGTRK media group and illegally crossing the border to Russia, although she claims she was abducted.

One complication for Poroshenko is the boost her release can give the opposition. As well as being regarded as a symbol of Ukrainian resistance to Russian aggression, Savchenko is also a Ukrainian lawmaker for the Batkivshchyna party of former prime minister Yulia Tymoshenko, who quit the ruling coalition in February and went into open opposition to the government after Groysman's appointment on April 18.

Poroshenko's media office said the president told Putin that "Ukraine urges Russia" to free Savchenko immediately. The fate of the two jailed Russian servicemen was discussed in the call, it confirmed in a statement.

The presidents also agreed to grant Ukrainian diplomats in Russia immediate access to Savchenko, who is in a poor state of health after a hunger strike that she ended on April 19. "Taking into account severe deterioration of Savchenko's health conditions, the president of Ukraine also urged to provide access to her for Ukrainian and German doctors," the statement said.