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.

Trump said his latest call with Putin wouldn't lead to "immediate peace" as he relayed the Russian leader's plans to retaliate against Ukraine.

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