U.S. President Donald Trump says he spoke with Russian President Vladimir Putin on Wednesday and that the Russian leader told him “very strongly” that he will respond to Ukraine’s weekend drone attack on Russian airfields.

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.

New satellite images and drone footage show serious damage inflicted on aircraft at several Russian airbases during Ukraine's surprise drone strike on Sunday. The images of two Russian airbases in north-western and central Russia, taken on Wednesday morning, show 12 aircraft damaged or destroyed.