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.
Mr. Medvedchuk’s close ties to Mr. Putin are no secret, and he was in Belarus on Tuesday 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 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 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.
“He is a friend of Putin and he is trying to find his place in the new Ukrainian-Russian environment,” Mr. Lozhkin said in the interview. “He is trying with hostages. Maybe it will be effective. And if it will be effective, I think it’s O.K. if Medvedchuk will do it.”
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He sat next to his old boss, Mr. Kuchma, whom Mr. Poroshenko had named as his official — and sole —representative in the negotiations.
Mr. Medvedchuk, in a message posted on Facebook, said he did not represent the rebels. Russia, in turn, had sent its own delegate.
Stanislav Belkovsky, a Russian political analyst, said that he had no doubt about Mr. Medvedchuk’s role: “He is Putin’s personal representative.”
Andrew Roth contributed reporting from Donetsk, Ukraine.
