Poland will increase the combat readiness of some troops and is considering activating Article 4 of the NATO military alliance treaty, officials said, following the deaths of two people in an explosion near the Ukrainian border.

Authorities did not say what caused the blast, but Polish media reported earlier in the day that missiles or rockets had struck the area.

Poland is in touch with NATO partners and US counterparts about the incident, government spokesman Piotr Muller said, and teams of experts will work on all night to clarify details of what he called a “serious” situation. “Poland is increasing the combat readiness of the army and other services,” he added.

It remains unclear where the projectiles came from, but they landed in the NATO member’s territory roughly the same time as Russia launched its biggest wave of missile attacks on Ukrainian cities in more than a month.

At the site of the explosion in the Hrubieszów district of eastern Poland, local media showed an image of a deep impact and upturned farm vehicle, near the town of Przewodow, around four miles west from the Ukrainian border. CNN cannot independently confirm the photos.