Russian President Vladimir Putin told US President Donald Trump in a phone call Wednesday that Moscow was obligated to respond to Ukraine’s weekend drone attack, Trump said, setting up a potential escalation in the conflict just as the US president hopes to broker an end to the war.

Putin in the call with Trump vowed retaliation against Ukraine for its recent drone attack on Russian bombers, the U.S. president said.

Ukrainian President Volodymyr Zelenskyy on Wednesday dismissed Russia’s ceasefire proposal as “an ultimatum” and renewed his call for direct talks with Russian President Vladimir Putin to break the deadlock over the war,