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.

President Donald Trump said Wednesday that Vladimir Putin intends to retaliate “very strongly” against Ukraine for a surprise drone strike on Russian airfields over the weekend, a declaration that provoked alarm in Washington and abroad even as Trump touted his efforts to bring peace to the region.

As the Russian president slow-walks peace talks, GOP leaders hope to pressure their own president into taking a tougher stance.