Countries around Ukraine have feared for months that the fighting could spill over their borders. On Tuesday, leaders in Poland saw an explosion on their soil as Russian forces launched one of the broadest attacks yet. Two people were killed.

Polish media reported the explosion in the village of Przewodow, about 25 kilometers from the border with Ukraine. This would be the first time since the war started that a Russian weapon has come down on a NATO country.

Polish leaders raised the readiness of military units. Government Spokesperson Piotr Mueller said, "At the same time, we have decided that we need to verify if there are grounds to invoke Article 4 of the NATO treaty."

Member countries use that mechanism when they feel threatened, and NATO leaders meet to decide on a response.

The explosion came as Russian forces fired about 100 missiles in strikes across Ukraine. However, Russian defense officials deny hitting Polish territory and described those suggestions as a "deliberate provocation."

Ukrainian President Volodymyr Zelenskyy called the strike "an attack on our collective security." He described it as a "very significant escalation and added that "we must act."

US diplomats say the reports are "incredibly concerning." They say they are still trying to determine what happened and what to do next.