A meeting between Mr. Putin and Mr. Zelensky would have been the first between the two men since before Russia invaded Ukraine three years ago, launching a war that by conservative estimates has resulted in well over one million dead and injured soldiers on both sides.

The proposed negotiations in Istanbul would be the first direct peace talks between Moscow and Kyiv in three years.

U.S. Secretary of State Marco Rubio and his NATO counterparts meet in Turkey to prepare a pivotal summit of alliance leaders next month that will set the course for future European security as America focuses on challenges elsewhere.