The diplomatic effort coincided with a Russian claim that its troops have driven the Ukrainian army out of a key town in Russia's Kursk border region.

President Vladimir Putin's top foreign policy aide said on Thursday a 30-day ceasefire proposed by the United States to pause the war in Ukraine would give Russia "nothing" while gifting Kyiv's forces a much-needed battlefield respite.

Why it matters: The U.S. and Ukraine both endorsed an unconditional 30-day ceasefire on Tuesday that calls for all air and artillery strikes to stop and fighting to cease all along the front lines. Putin raised doubts about that idea, and said any ceasefire should also be part of a process that addresses the "root causes of this crisis."