KIEV, Ukraine — Viktor V. Medvedchuk often turns up in places where only buddies of President Vladimir V. Putin of Russia get to go, like the V.I.P. section at a Formula One championship in Sochi, Russia, in October. He sat just a few feet from Mr. Putin, next to the actor Steven Seagal, another pal of the Kremlin leader’s.

Mr. Medvedchuk’s close ties to Mr. Putin are no secret, and he was in Belarus on Wednesday for negotiations between the Ukrainian government and Russian-backed separatists waging war in eastern Ukraine. But in a sign of the inscrutable nature of Ukrainian politics, he was there not on behalf of the Russian leader or the separatists but for President Petro O. Poroshenko of Ukraine and the Ukrainian state security service.

In an interview, Boris Lozhkin, the head of Mr. Poroshenko’s presidential administration, said that what matters most is what Mr. Medvedchuk — who has participated in the negotiations since they started last June — can contribute to ending the conflict in eastern Ukraine.

The security service, known by its Ukrainian abbreviation S.B.U., also said recently that it had enlisted the services of Mr. Medvedchuk in helping negotiate an exchange of hostages, designating him the agency’s special envoy for humanitarian issues.