"Here, you know, it would be difficult for both Poland and any NATO or other Western country to be ready to react immediately," he told reporters on Thursday. "It would certainly be complicated. We don't have that understanding, and nobody ever has, of having an entire country under the air defense umbrella."
Rupsys also noted that one needs intelligence data to be prepared for such a situation, adding that it would be difficult to react immediately without it.
Lithuania faces similar threats because of having a border with Belarus, the army chief said.
Two people were killed when a missile landed in a village near its border with Ukraine on Tuesday evening.
Washington and Warsaw believe it was fired by Ukrainian forces defending against Kremlin missile attacks, but the end responsibility for the incident still lies with Moscow, the West believes.
For its part, Kyiv believes the missile was launched by Russia and wants access to "all the available data".
