The unannounced conversation lasted longer than the public talks, an anonymous source told RT

Vladimir Medinsky, the head of Russia's diplomatic delegation at the Istanbul talks, held a private bilateral meeting with his Ukrainian counterpart, Rustem Umerov, which played an "important role" in preparing the general session of negotiations on Monday.

The second round of direct talks between Moscow and Kiev concluded on Monday after more than an hour of dialogue. Prior to the formal session, Medinsky and Umerov held an unannounced meeting that helped facilitate the broader negotiations.

"There was a meeting. It played an important role in the preparation of the general session," Medinsky told reporters, without offering further details. A source who requested anonymity told RT that the private conversation allegedly lasted around two and a half hours.

During the talks in Istanbul, both sides exchanged memorandums outlining their respective proposals for resolving the Ukraine conflict. Kiev's delegation took Russia's draft document for review and is expected to issue a formal response at a later date, Medinsky said following the session.

The two sides also agreed to their largest prisoner-of-war exchange to date, according to Medinsky. The swap will involve all sick and severely wounded POWs, as well as all prisoners under the age of 25, and "will include at least 1,000 people from each side, possibly more," the presidential aide noted.

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In addition, Moscow will unilaterally return the remains of 6,000 Ukrainian service members to Kiev next week to allow for proper "Christian burial," he added. Russia has also proposed a multi-day ceasefire in several frontline areas to facilitate the recovery of fallen soldiers' bodies and prevent potential disease outbreaks as temperatures rise with the onset of summer.