Warsaw says a Russian-made missile has hit a village near the Ukraine border, killing two people and fueling concern over what the incident will mean for all NATO allies.

Poland says it confirmed the blast was caused by a missile, but said there was no concrete evidence of who fired it.

If it was Russia, it would be Moscow's first strike on a NATO member.

That has led to fears that NATO could invoke Article 5, which holds that an attack against one member is considered an attack against all members, triggering a war with Russia.

Poland is reportedly considering invoking Article 4 of the security alliance's treaty, which calls for consultations if a member country feels its territorial integrity is threatened.

NATO Secretary General Jens Stoltenberg tweeted that the alliance is monitoring the situation and it's "important that all facts are established."

Allies are coming forward to offer early support. US President Joe Biden spoke to his Polish counterpart and pledged assistance in the investigation.

Ukraine's President Volodymyr Zelenskyy called what happened in Poland a "significant escalation" and says it was foreseeable.

Zelenskyy said, "What we warned about a long time ago happened today. We've been telling this. Terror is not limited to our national borders. It has already spread to the territory of Moldova. And today, Russian missiles hit Poland."

Russian defense officials have denied Russian missiles struck Polish territory. They called the accusation a "deliberate provocation."