Russian President Vladimir Putin told President Donald Trump in a phone call Wednesday that he 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.

In the aftermath of Ukraine’s drone attack on an air base, President Donald Trump and Russian President Vladimir Putin recently spoke over the phone for an hour and 15 minutes. Trump says Putin told him "very strongly" Russia will have to respond to the attack from Ukraine.