The Kremlin said on Wednesday it would review details from Washington about a proposal for a 30-day ceasefire in Ukraine before responding, while U.S. Secretary of State Marco Rubio hoped a deal would be struck within days.

As Moscow considered the plan, President Vladimir Putin, dressed in military fatigues, made a surprise visit to Russia's Kursk region for the first time since Ukrainian troops captured part of it last year.

With Putin's presence highlighting recent Russian advances in Kursk, Valery Gerasimov, head of Russia's General Staff, told the Kremlin leader his troops had repelled Ukrainian forces from 86% of the ground they once held in Kursk. Ukraine had hoped to use that territory as a bargaining chip in any peace talks with Moscow.