US President Joe Biden told G7 and NATO partners that a missile blast in eastern Poland was caused by a Ukrainian air defense missile, a NATO source told Reuters on Wednesday.

The blast, which killed two people, raised global alarm that the Ukraine conflict could spill into neighboring countries.

For its part, Russia said that its Tuesday strikes were carried out on targets only on the territory of Ukraine, saying its military experts identified the rocket as fragments of a guided anti-aircraft missile of a Ukrainian S-300 air defense system, according to AFP.

The Russian Defense Ministry said in a statement that strikes were carried out at a distance of no closer than 35 kilometers from the Ukrainian-Polish border.

“Photographs of the wreckage… were unequivocally identified by Russian military experts as fragments of a guided anti-aircraft missile of a Ukrainian S-300 air defense system,” the statement added.

Russian Spokesman Dmitry Peskov said that the American response had been “restrained” compared to that of other countries, stressing that Russia had nothing to do with the incident.