Poland is putting some military units on a heightened state of readiness after missiles reportedly fired by Russia landed in the country, a government spokesman said on Tuesday, according to the AFP news agency.
"There has been a decision to raise the state of readiness of some combat units and other uniformed services," spokesman Piotr Muller told reporters after an emergency national security council meeting in Warsaw.
Two people were reportedly killed after the missile hit an area in Przewodow, near the Polish border with Ukraine.
The United States and Western allies said they were investigating but could not confirm a report that the blast in Poland resulted from stray Russian missiles, as Russia's defense ministry denied it.
Members of the North Atlantic Treaty Organization (NATO) are committed to collective defense, so a Russian strike on Poland could risk widening the conflict between Russia and Ukraine, which began with Moscow's invasion in February.
In Washington, however, the Pentagon, White House and US State Department said they could not corroborate the report and were working with the Polish government to gather more information, according to Reuters.
The State Department said the report was "incredibly concerning."
Germany and Canada said they were monitoring the situation, and Norway said it was seeking details.