NATO envoys gather as the explosion in Poland alarms the alliance.

Military analysts noted that both the Russian and Ukrainian militaries could be using Russian-made missiles, leaving open a number of possible causes for the explosion. It could have involved a Russian missile that flew off course or was knocked off its trajectory by an intercepting Ukrainian air defense missile, or a missile fired by Ukraine to shoot down an incoming Russian strike.

Turkey’s president, Recep Tayyip Erdogan, said that he believed “this has nothing to do with Russia.” He told a news conference in Bali: “Maybe this is a technical mistake, or any other explanation will be found.”

The United Nations secretary general, António Guterres, said he was “very concerned” by the explosion. “It is absolutely essential to avoid escalating the war in Ukraine,” he said in a statement.

Since the beginning of the war, the United States and its allies have sought to keep the fighting limited to Ukrainian territory and to avoid direct confrontation with Russia, even as NATO members have supplied a steady stream of weapons to Kyiv.

But if the explosion is determined to have been a deliberate attack, it could have broad consequences. Article 5 of the NATO charter commits its members to mutual defense, stating that an attack on one is an attack on all. That could be taken as requiring a concerted response to the blast in Poland. President Volodymyr Zelensky of Ukraine, eager for more NATO support, said Tuesday that Russia had committed an “attack on collective security,” hinting at Article 5.

Under Article 4 of the charter, any member country can request a formal consultation among all members on an issue of concern. A spokesman for the Polish government had said that it was considering invoking the provision.

Late on Tuesday, the U.S. defense secretary, Lloyd J. Austin III, and the secretary of state, Antony J. Blinken, spoke with their Polish counterparts. Mr. Austin assured Poland’s defense minister “of the ironclad commitment of the United States to defend Poland,” according to a statement provided by the Pentagon.

Reporting was contributed by Farnaz Fassihi, Edward Wong, Eric Schmitt, Chris Buckley and Jim Tankersley.

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