Initial reports blamed Russian missiles for the explosion, which killed two people and raised fears of a major escalation of the Ukraine conflict, but Poland later said it was likely caused by a stray surface-to-air missile fired by Kyiv's forces.

"We'll continue to work closely with our ally Poland and others to gather more information, and we'll continue to consult closely with our NATO allies and our valued partners," Austin said as he opened a virtual meeting of the Ukraine Defense Contact Group, which is made up of dozens of countries that back Kyiv.

"What we do know is the context in which this is unfolding. Russia is facing setback after setback on the battlefield, and Russia is putting Ukrainian civilians and civilian infrastructure in its gunsights," he added.

NATO chief Jens Stoltenberg, who was also to take part in the Ukraine contact group meeting, was more direct in blaming Russia, saying: "This is not Ukraine's fault."

"Russia bears ultimate responsibility as it continues its illegal war against Ukraine," he said after chairing a meeting of NATO ambassadors.

The explosion in eastern Poland came as Russia launched dozens of missile strikes across Ukraine on Tuesday, which Kyiv said knocked out power to millions of people.

Building multilayer air defenses to protect against such strikes is a key goal of Ukraine's allies, who have provided various missile systems to Kyiv.

Austin on Wednesday said the advanced NASAMS systems Washington has sent are now in operation in Ukraine.

"Our NASAMS air defense systems are now operational, and they have had 100 percent success rate in interrupting Russian missiles as the Kremlin continues its ruthless bombardment of Ukraine, including yesterday's attacks," he said.