The protracted negotiations over Wednesday’s prisoner swap involved Viktor V. Medvedchuk, a Ukrainian with close ties to Russia’s president, Vladimir V. Putin. The Russian leader, who is up for re-election in March, saw his popularity ratings in Russia soar after he grabbed Crimea but he has garnered no discernible political benefit from Russia’s intervention in eastern Ukraine.

The rebels, an unruly group of feuding gunmen and fervently pro-Russian political operatives, have shown no interest in a long-term settlement that would strip them of Russian support, without which their secession movement would probably crumble. Mr. Putin, anxious about alienating hard-line nationalists in Russia, seems disinclined to cut the rebels loose but faces rapidly diminishing returns from a military venture that has poisoned Moscow’s relations with the West.

Speaking in Vienna earlier this month, at a meeting of the Organization for Security and Cooperation in Europe, whose monitoring mission in eastern Ukraine recently reported a sharp uptick in fighting, the American secretary of state, Rex W. Tillerson, said the conflict “stands as the single most difficult obstacle to us renormalizing the relationship with Russia, which we badly would like to do.”

While Ukraine’s president, Petro O. Poroshenko, rejoiced at the exchange of prisoners and showed up in military fatigues near rebel-held territory to meet the released Ukrainians, the authorities in separatist-held Donetsk responded with accusations that Ukraine had violated its obligations. The Donbass News Agency, a rebel mouthpiece, made a litany of complaints, accusing Ukraine of not blocking certain roads as agreed and not living up to the spirit of the Minsk deal.

Ukraine was originally supposed to return 306 prisoners to rebel-held territory but said that 40 of these had already been released and did not show up for the exchange, carried out at checkpoint near the rebel-held town of Horlivka. A further 29 captives held by Ukraine refused to return to rebel-held territory, Ukrainian officials said.