President Joe Biden on Thursday disputed claims made by Ukraine’s leader about the origin of the missile that struck Poland earlier this week, after initial findings painted the incident that at first sparked international panic as likely as an accident.

NATO’s top official and Poland’s president had both said on Wednesday that early stages of the investigation show the strike – which killed two people in Przewodów, Poland – was likely caused by Ukrainian air defense systems while protecting against Russian rocket attacks. But Ukrainian President Volodymyr Zelenskyy sowed doubt in that analysis.“I don't know what happened,” Zelenskyy said on Thursday, according to a translation on his official website. “We don't know for sure. The world does not know. But I am sure that it was a Russian missile, I am sure that we fired from air defense systems. But it is impossible to talk about something specific today – that it was the air defense of Ukraine. Although we are grateful that no one accuses us, because we are fighting against Russian missiles on our territory.”

Returning to the White House early Thursday morning after an overseas trip, Biden said in response to a question about Zelenskyy’s comments: “That's not the evidence.”

NATO Secretary General Jens Stoltenberg told reporters following a meeting of the alliance’s North Atlantic Council on Wednesday – called due to the deadly strike – that Russia still “bears ultimate responsibility” because of its “illegal war” waged against Ukraine. The rocket does appear to have been a Russian-made S-300, according to Polish President Andrzej Duda. Russia denied initial allegations that its armed forces were responsible for the blast.The investigation is ongoing. Zelenskyy said Ukraine’s request to take part in it was approved, noting Ukrainian “specialists will go there and join this professional community.”

Biden and Zelenskyy’s disagreement over the missile is not the first time there have been signs of friction between the two. NBC News has reported that a June call between the leaders included Biden losing his temper with Zelenskyy for not showing more gratitude over the $1 billion in U.S. military assistance being provided to Ukraine at the time. But their relationship has improved since then, according to the outlet, which attributed its reporting to administration officials.

Biden’s comment on Thursday is the only one from a U.S. official directed at Zelenskyy’s remarks about the explosion specifically thus far, and the administration remains united in its support for Poland’s findings.

“We have full confidence in the Polish government’s investigation of the explosion near their border with Ukraine, and we commend them for the professional and deliberate manner in which they are conducting it,” Adrienne Watson, spokeswoman for the National Security Council, said in a Wednesday statement. “We have seen nothing that contradicts President Duda’s preliminary assessment that this explosion was most likely the result of a Ukrainian air defense missile that unfortunately landed in Poland.”