NATO Secretary General Jens Stoltenberg said the missile that hit Poland on Tuesday was likely Ukrainian.
He told the BBC it was probably a Ukrainian anti-aircraft missile.
Stoltenberg said the investigation into the explosion near the border with Ukraine was continuing.
But he insists Russia is ultimately responsible for the ongoing aggression in Ukraine.
Ukraine itself has been saying that Russia fired the missile.
President Volodymyr Zelensky said in televised remarks, ‘I have no doubt that this is not our missile. Based on our military reports I believe it was a Russian missile.
He said it was essential to allow Ukraine to join the investigation into the blast, 6 kilometers from the border.
Ukrainian air defenses were activated on Tuesday as Russia launched its biggest wave of missile strikes in nine months since the February 24 attack.
Dozens of Russian missiles were fired at the country. But Ukraine says it has managed to bring down most of them.
The mass attack during the G20 summit in Indonesia has caused an international outcry. News of a missile explosion inside the territory of NATO member Poland has sparked fears the war could escalate dangerously.
Stoltenberg said NATO had promised to provide more advanced air defense systems to Ukraine, which is not a member of the alliance but receives extensive military assistance.
“Today I attended a support group meeting for Ukraine, where NATO allies and partners made new commitments for more advanced air defense systems to help us shoot down Russian missiles,” the NATO chief said.
The best way to prevent such incidents in the future is for Russia to stop fighting, he said.
“We have no indication that this was a deliberate attack by Russia,” he said from NATO headquarters in Brussels.
But he said there is no doubt that Russia is responsible, because if Russia had not launched a barrage of missile attacks on Ukrainian cities yesterday, as they have done many times before during this war, this would not have happened.
Earlier, Polish President Andrzej Duda said that although it was blamed on the Russian-made S-300 missile, there was no evidence that it was launched by Russia.
(Dak Times/17 November/FA)