Ukrainian President Volodymyr Zelenskyy has continued to maintain that he has no doubt that an explosion that killed two people in Poland was not caused by a Ukrainian missile.
It puts him at odds with Western allies who concluded on Wednesday that the origin of the blast was Ukrainian air defence as it tried to down Russian missiles. But all have blamed Russia for being ultimately responsible.
"I have no doubt that it was not our missile," Zelenskyy was quoted as telling Ukrainian media.
He said he believed Tuesday's explosion was caused by a Russian missile, adding that he based his conclusions on reports from Ukraine's military which he "cannot but trust".
US President Joe Biden on Thursday disputed the Ukrainian leader's comment, upon returning from a trip to Asia where he attended the G20 summit.
"That's not the evidence," Biden told reporters at the White House.
NATO and Poland also concluded that the missile that crashed in Poland was probably a stray fired by Ukraine's air defences.
Nevertheless, NATO's chief said that Russia, not Ukraine, was still to blame for starting the war with its February invasion and launching scores of missiles on Tuesday that triggered Ukrainian defences.
The incident occurred while Russia unleashed a barrage of missiles on cities across Ukraine, targeting its energy grid and worsening power blackouts for millions, in what Ukraine says was the most intense bombardment of the nine-month-long war.
"This is not Ukraine's fault. Russia bears ultimate responsibility as it continues its illegal war against Ukraine," NATO Secretary-General Jens Stoltenberg told reporters in Brussels.
NATO ambassadors held emergency talks on Wednesday to respond to Tuesday's blast that killed two people at a grain
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