A day after hopping on the phone with Russian President Vladimir Putin and warning him of potential “widespread human suffering” if Moscow invaded its neighbor, U.S. President Joe Biden had an hour-long call with Ukrainian President Volodymyr Zelensky, in which he assured the leader the U.S. would respond “swiftly and decisively” in case of invasion. According to a White House readout of the call, Biden and Zelensky “agreed on the importance of continuing to pursue diplomacy and deterrence in response to Russia’s military build-up on Ukraine’s borders.” The last time the two leaders spoke it “did not go well,” according to a senior Ukrainian official who spoke to CNN. At that time, the two apparently disagreed on the level of risk of a Russian invasion.