NATO Secretary General Jens Stoltenberg said Wednesday a deadly explosion in Poland was probably the result of Ukrainian anti-aircraft fire but that Russia bears “ultimate responsibility” for the war.

“An investigation into this incident is ongoing, and we need to await its outcome…. But we have no indication that this was the result of a deliberate attack,” Stoltenberg said after chairing a meeting of NATO ambassadors.

“Our preliminary analysis suggests that the incident was likely caused by a Ukrainian air defence missile fired to defend Ukrainian territory against Russian cruise missile attacks.

“But let me be clear, this is not Ukraine’s fault,” he continued.

“Russia bears ultimate responsibility as it continues its illegal war against Ukraine.”

Russia launched a wave of missile strikes against Ukraine on Tuesday. It insists it is not to blame for the blast on Polish territory which triggered fears the conflict could spread.

Poland called the emergency meeting of NATO in Brussels, sparking concern the NATO allies might be coming closer to open confrontation with Russia.

But Warsaw’s own President Andrzej Duda has also said of the explosion, which killed two in a Polish village near the Ukrainian border, that “very likely that it was a rocket used in anti-missile defense, meaning that it was used by Ukraine’s defence forces.”

Stoltenberg said that NATO had ramped up its defences along its eastern flank in response to the war in Ukraine and denied the alliance’s air defences had failed.

“We are constantly assessing what more we need to do,” Stoltenberg said.

“We are prepared to handle situations like this in a firm, calm, resolute way, but also in a way that prevent further escalation.”

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He said NATO members were looking to create a “layered” air defence system for Ukraine by giving Kyiv more capabilities to shoot down cruise and ballistic missiles and drones.

“That’s exactly what the allies are providing in different ways,” he said.

The NATO chief said that Poland had stopped short of invoking the alliance’s Article 4 for further urgent talks over any perceived threat to its territory.

“That’s based on the findings based on the analysis and based on the results so far of the ongoing investigation.”