"In addition to prosecutors, military experts, police, gendarmes and representatives of SKW (Poland's military counter-intelligence agency) are at the scene," Justice Minister Zbigniew Ziobro said on Twitter.

It landed in a wood "a dozen or so kilometres from Bydgoszcz," the minister added. The location is far from the Russian and Ukrainian borders.

In a separate tweet, the defence ministry said, "the situation does not threaten the safety of residents" of Zamosc, a nearby village.

The regional prosecutor's office said in a statement that there were no "signs of an explosion or any explosives at the scene", adding that various hypotheses were being looked into.

Local media said the object was found earlier this week and is believed to be an air-to-ground missile.

RMF FM radio tweeted that it is "several metres long and stuck straight into the ground... The tube was empty... The warhead was also missing".

The station said the object bore Cyrillic writing though a military expert it quoted recalled that Poland's own army has a lot of post-Soviet arms.

"It did not necessarily fly in from Russia or the battle lines" in war-torn Ukraine, said Roman Polko, a former commander of the GROM Polish special forces unit.

He said the object could have come from military exercises being held "for the Ukrainians or for Polish pilots".