MOSCOW — Leaders of Russia, Turkey and Iran announced an agreement on Wednesday to sponsor a conference aimed at achieving a peaceful settlement of the Syria war.

The announcement by the three leaders, who were meeting in the Russian Black Sea resort of Sochi, underscored Russia’s increasingly influential role in determining the outcome of the war, which appears to be winding down after nearly seven years.

The conference they proposed, also to be held in Sochi, was announced just days before a new round of peace talks was planned in Geneva, convened by a special United Nations envoy, Staffan de Mistura.

It was another sign that Mr. de Mistura’s efforts, frustrated for years, have now been overshadowed by Russia’s actions.