"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, triggering concern that 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 defence, meaning that it was used by Ukraine's defence forces".