• Ukraine's Defense Ministry was cautious Wednesday as evidence mounted that its own armed forces fired a missile that hit Poland.
• Several media reports cited Western officials as saying initial assessments suggested the missile was fired from a Ukrainian air defense system.
• Ukraine's Defense Ministry said the issue was "very sensitive" and welcomed a thorough investigation.

Ukraine's Defense Ministry responded cautiously to mounting evidence suggesting its own armed forces fired a missile that hit Poland, killing two people — saying the issue was "very sensitive" and that it wanted its own officials to be able access the site where the incident took place.

Early Wednesday morning, The Associated Press reported, citing three unnamed U.S. officials, that preliminary assessments indicated "the missile that struck Poland had been fired by Ukrainian forces at an incoming Russian missile."

Other media agencies, including NBC News, cited similar details on Wednesday; Reuters reported a NATO source as saying President Joe Biden had told the G-7 and NATO partners that the strike was caused by "a Ukrainian air defense missile," while The Wall Street Journal cited two senior Western officials briefed on the preliminary U.S. assessments as saying the missile was from a Ukrainian air defense system.

Those initial findings were then confirmed by NATO on Wednesday morning with Secretary-General Jens Stoltenberg telling reporters that while investigations continue, the strike was likely caused by a Ukrainian air defense missile but that, ultimately, Russia was "responsible for the war that has caused this situation."

Earlier Wednesday, Ukraine's Defense Ministry was cautious about the initial assessments of the incident. Yuriy Sak, an advisor to Ukraine's defense minister, Oleksiy Reznikov, told CNBC that Kyiv welcomed a thorough investigation of the incident and said the issue was "very sensitive."

"It is too early to give any definitive answers and it's very dangerous to jump to any conclusions," Sak said Wednesday morning.

"I would like to just stress once again that right now, the president of Poland has said that there are no conclusive evidence of what exactly has happened. Joe Biden, when he was making his comment, he was also cautious because everybody understands that this is a very sensitive issue," he said.

"Before any conclusions are made, an investigation must be done. So, that is where we stand," he said.

After NATO's comments, Oleksiy Danilov, head of Ukraine's National Security and Defense Council, tweeted that Kyiv favored a "joint study" into the incident. Danilov said on Twitter that Ukraine was "ready to hand over the evidence of the Russian trail that we have" but Kyiv was still awaiting "information from our partners, on the basis of which a conclusion was made that it is a Ukrainian air defense missile."

He added that Ukraine had requested that Defense Ministry and border guard officials be granted immediate access to the site of the explosion.

Missile hit context

Tuesday night's incident came after Ukraine suffered a wave of missile strikes by Russia with one Ukrainian official saying more than 90 missiles were fired at the country. The attacks knocked out energy infrastructure across Ukraine, reportedly leaving 7 million people without power.

For its part, Ukraine blamed Russia for the missile that hit Poland on Tuesday night, with President Volodymyr Zelenskyy reportedly telling his Polish counterpart that it was "a rocket launched from the territory of the Russian Federation." Russia said it had not fired the missile and called it a "deliberate provocation in order to escalate the situation."

Ukrainian defense official Sak told CNBC that Ukraine's international allies should have responded to Kyiv's repeated requests for them to impose a no-fly zone over Ukraine.

NATO refused to do that early in the war, fearing it would be dragged into a direct conflict with nuclear power Russia.

"What we want to stress is that if there was no invasion of Ukraine, yesterday would not have happened. If the Ukrainian sky would have been closed at our request by our allies, this would not have happened," Sak said, echoing comments by British Prime Minister Rishi Sunak who said Wednesday morning that "none of this would be happening if it wasn't for the Russian invasion of Ukraine."

Sak said it was crucial that the missile incident didn't distract from Ukraine's defense needs.

"It is very important that we don't shift the focus now and that we continue to discuss the options for further closing the Ukrainian sky, providing Ukraine with efficient air defense systems, because what needs to happen is that we need to all collectively make sure that such tragic incidents as yesterday do not happen again," he said.

As a flurry of urgent and high-level diplomatic talks are taking place among NATO members on Wednesday, defense analysts agreed that, whether Russia fired the missile or not, it bears a lot responsibility for the attack.

"Russia is to some degree culpable regardless, because it's firing missiles on civilian infrastructure targets, and firing them dangerously close to NATO territory and the Ukrainian-Polish border, and Ukraine needs to defend itself," Samuel Ramani, a geopolitical analyst and associate fellow at the Royal United Services Institute defense think tank, told CNBC on Wednesday.

"But it may not be that Russia intentionally targeted Poland, and it could be Ukraine doing it. So right now, I think we need an investigation to figure out what's really happening."