Poland's president on Wednesday said a missile strike within the border it shares with Ukraine was an "unfortunate accident" and not a deliberate Russian attack as originally feared.

Authorities believe the missile was fired by Ukraine.

"Ukraine's defense was launching their missiles in various directions and it is highly probable that one of these missiles unfortunately fell on Polish territory," President Andrzej Duda said. "There is nothing, absolutely nothing to suggest that it was an intentional attack on Poland."

Poland is a member of NATO and there was concern a Russian provocation there could broaden the war to include NATO member countries coming to Poland's defense.

A missile struck a grain silo and killed two Polish citizens about 15 miles inside the Polish border. Initial reports blamed Russia, ratcheting up tensions in the region.

NATO Secretary-General Jens Stoltenberg said it appears a Ukrainian air defense missile was attempting to intercept an incoming Russian missile. Nonetheless, he said Russia is still ultimately at fault.

"Let me be clear: this is not Ukraine's fault. Russia bears ultimate responsibility as it continues its illegal war against Ukraine," Stoltenberg said.

Russian Defense Ministry spokesman Igor Konashenkov said Russian officials had studied images of the explosion in Poland and concluded rocket fragments were from an S-300 surface-to-air missile system fired by Ukraine, NPR reported.