Polish officials on Tuesday said that two people died following an explosion in the Hrubieszów district in eastern Poland.

“There was an explosion in eastern Poland,” said government spokesman Piotr Muller said in Warsaw. He said experts were gathering details on what he called a “serious” situation.

Poland is considering asking its NATO partners for discussions under Article 4 of the NATO Treaty.

Article 4 allows for any member to seek consultations with the rest of the Alliance, and states: “The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened.”

Kyiv’s response: Ukrainian President Volodymyr Zelensky said it was Russian missiles that hit Poland. However, Russia’s defense ministry said no strikes were made “on targets near the Ukrainian-Polish state border.”

CNN at the scene: CNN’s Matthew Chance is in the small town of Przewodów, close to where a fatal explosion occurred near the Polish-Ukraine border. Chance said police have sealed off the road and journalists are not being permitted to go to the site. A local resident told CNN they heard a terrifying “whoosh” as a projectile flew over the town. Chance spoke to a caretaker of a local school who said the force of the explosion shook classroom windows approximately 200 meters from the school gates. Students were not inside the school when the explosion occurred. 

Large smoke plume visible: New videos and photos show a large smoke plume and a crater in Przewodów, which is just 4 miles from the Ukrainian border. Video taken by a resident shows a large smoke plume in the center of the village. CNN geolocated and confirmed the authenticity of the video.

Here’s how world leaders are reacting: US President Joe Biden spoke by phone with President Andrzej Duda of Poland from Bali, Indonesia, where he was attending the Group of 20 Summit, the White House said. NATO Secretary General Jens Stoltenberg has said that it is “important that all facts are established,” after speaking with Duda about the explosion. Meanwhile, French President Emmanuel Macron and EU Council President Charles Michel proposed a meeting with fellow EU leaders attending the G20 summit.