Two Polish citizens died in an explosion near the border with Ukraine on Tuesday, Polish authorities said after an emergency meeting of the country’s national security and defense council.

The cause was under investigation, a government spokesman, Piotr Mueller, said. But the explosion — on a day that Russia launched a broad missile attack on Ukraine, including territory just across the border from the site of the explosion — raised anxiety as local reports suggested Russian weapons were to blame, a prospect that could have broader consequences because of Poland’s membership in NATO.

At a Pentagon briefing in Washington, Brig. Gen. Patrick S. Ryder said that the Defense Department was aware of media reports saying that two Russian missiles had landed in Poland, but said that U.S. defense officials had no corroborating information.

Two diplomats from NATO countries, who spoke on condition of anonymity because they were not authorized to speak to reporters on the issue, said that NATO ambassadors would meet in Brussels on Wednesday morning to discuss the situation. Separately, a top E.U. official, Charles Michel, urged European Union leaders attending a Group of 20 summit in Bali — including those from Germany, France and Italy — to hold a “coordination meeting” there on Wednesday over the blast in Poland.

President Biden, who is at the G20 meeting, has been briefed on the reports out of Poland and spoke with President Andrzej Duda of Poland, the White House said.

The explosion occurred in the village of Przewodow, about four miles north of the Ukrainian border.

Russia’s Defense Ministry denied any involvement. On Telegram, the ministry wrote that any statements by Polish officials or media outlets about Russian missiles hitting the village were a “deliberate provocation.”

“No strikes on targets near the Ukrainian-Polish state border were made,” the ministry wrote.

Mr. Mueller, the Polish government spokesman, said special procedures had been put in place, including boosting combat readiness of certain military units. He said the Polish government was also examining the possibility of triggering Article 4 of the NATO charter, under which members can consult with one another when a nation feels its territorial integrity or security has been threatened.

Russia launched a widespread missile attack on Ukraine on Tuesday, with roughly 90 missiles aimed primarily at the country’s electrical infrastructure. Ukraine’s Volyn region, where Russian rocket strikes were reported on Tuesday, lies across the border from Przewodow.

The explosion’s proximity to the border raised the possibility it could be the result of an errant missile, or the remains of one that had been targeted by Ukraine’s air defense systems. The Ukrainian air force said 70 incoming Russian missiles had been shot down on Tuesday. A fragment of one destroyed missile struck a residential building in Kyiv, killing one person.

Ukraine’s foreign minister, Dmytro Kuleba, said in a Twitter post that the explosion in Poland was not caused by a Ukrainian air-defense missile.

Although the cause of the explosion remained unclear — including whether it involved munitions or was instead the result of some other cause — Ukraine’s president, Volodymyr Zelensky, seized on reports of possible Russian involvement, calling it evidence of “a very significant escalation.”

Mr. Zelensky alluded to Poland’s membership in NATO and blamed Russia for what he called an “attack on collective security.”

Since the beginning of the invasion, Ukraine’s Western allies, including the United States, have sought to keep the fighting limited to Ukrainian territory and avoid direct confrontation between the alliance and Russia, even as they have supplied a steady stream of weapons to Kyiv.

Jens Stoltenberg, the secretary general of NATO, said he had been in touch with Mr. Duda, and offered his condolences for the loss of life. He said NATO was monitoring the situation and allies were consulting with one another.

“Important that all facts are established,” he wrote on Twitter.

Vedant Patel, a U.S. State Department spokesman, said the United States was working with the Polish government and other members of NATO to gather more information.

Asked whether it would matter if any damage was found to be accidental or intentional, he said, “Of course that is something that would be important.”

Steven Erlanger, Julian E. Barnes, Richard Pérez-Peña, Michael Crowley and Carly Olson contributed reporting.