The blast set off tense moments with world powers as Russia’s war in Ukraine rages on.
The Polish government convened an emergency security and defense meeting Tuesday and agreed to increase its military readiness, government spokesman Piotr Mueller said, after an explosion killed two people near the border with Ukraine.
It was not clear what caused the explosion in the southeastern town of Hrubieszów, Mueller said after the emergency meeting was convened.
"A moment ago it was decided to increase the readiness of some military units in Poland and other uniformed services," he told reporters.
Unconfirmed news reports that the blasts were caused by Russian rockets hitting a town a few miles from Ukraine — whether intentional or by mistake — raised fears of an escalation in President Vladimir Putin’s war.
“We’ve seen these reports out of Poland and are working with the Polish government to gather more information,” the spokesperson for the National Security Council said in a statement. “We cannot confirm the reports or any of the details at this time. We will determine what happened and what the appropriate next steps would be.”
Russia pushed back on the allegations soon after the reports emerged.
“The statements of the Polish media and officials about the alleged fall of ‘Russian’ missiles in the area of the settlement of Przewodow, is a deliberate provocation in order to escalate the situation,” the Russian Ministry of Defense said in a statement on Telegram, referring to the village widely reported to have been hit.
The increasingly urgent situation comes as Russia launched an intense wave of airstrikes on cities across Ukraine on Tuesday, forcing widespread blackouts and hitting residential buildings in the capital, Kyiv.
The barrage targeted key cities from Lviv in the west to Kharkiv in the northeast, pounding energy infrastructure and knocking out power to vast areas in one of the largest coordinated attacks of the war.
“It is only a matter of time before Russian terror goes further. The longer Russia feels impunity, the more threats there will be to anyone within reach of Russian missiles,” Ukrainian President Volodymyr Zelenskyy wrote on Telegram. “This is a Russian missile attack on collective security! This is a very significant escalation. We must act.”
The prospect of Russian missiles possibly hitting Poland, a member of NATO, triggered fears that the war in Ukraine could intensify sharply.
“We can and should do everything possible to come to Poland’s defense,” U.S. Sen. Chris Coons, D-Del., told NBC News. “I hope the Russians will promptly take responsibility, identify this as a mistake, if it is in fact a mistake, provide compensation and commit to no more strikes within some buffer of the border.”
Poland, in theory, is covered by NATO’s Article 5 — a tenet that suggests that all allies, including the U.S., would come to the aid of any member that comes under attack.
Jens Stoltenberg, NATO's secretary general, tweeted that he "offered my condolences for the loss of life" to Polish leaders and that NATO is "monitoring the situation and Allies are closely consulting."
Some alliance members also expressed their support of Poland and said they were in contact with the country’s leaders.
Gitanas Nauseda, the president of Lithuania, which borders Poland, tweeted that “every inch of NATO territory must be defended!”
Czech Prime Minister Petr Fiala responded on Twitter that “if Poland confirms that the missiles also hit its territory, this will be a further escalation by Russia. We stand firmly behind our EU and NATO ally.”
Fellow NATO member Estonia, which for decades was part of the Soviet Union and has in recent years warned of Russian expansionism, called the news “most concerning.”
“We are consulting closely with Poland and other allies. Estonia is ready to defend every inch of NATO territory,” the country’s foreign minister said via Twitter. “We’re in full solidarity with our close ally Poland.”