Ukrainian President Volodymyr Zelenskyy said Monday he expected to discuss “key issues” with U.S. President Donald Trump as the American-led push to end Russia’s war on Ukraine accelerated, with Trump’s envoy heading to Moscow.

Ukraine’s negotiator, Rustem Umerov, said “significant progress” had been achieved in two days of talks with U.S. officials on Trump’s plan to end the war, even though more work was needed on “challenging” issues.

The comments came as U.S. envoy Steve Witkoff was due to meet Russian President Vladimir Putin in Moscow on Tuesday, the latest in a flurry of diplomacy to halt the conflict.