The remains of a "military object" were found in a forest in northern Poland on Thursday, which a broadcaster said authorities believed could be part of a missile stuck in the ground.

The defence and justice ministries did not identify the object found near the city of Bydgoszcz, or say how long it had been there.

Private broadcaster RMF FM reported that it was an air-to-surface missile measuring several metres, stuck in the ground, with its head missing. It identified no sources for its report.

Poland has been on alert for possible spillover of weaponry from the war in neighbouring Ukraine, especially since two people were killed near the border last November by what Warsaw concluded was a misfired Ukrainian air defence missile.

"The Military Department of the District Prosecutor's Office in Gdansk, under the supervision of the National Prosecutor's Office, initiated proceedings regarding the remains of an aerial military object found in a forest several kilometres from Bydgoszcz," Justice Minister Zbigniew Ziobro said on Twitter.

The defence ministry said on Twitter the remains of "an unidentified military object" were found in the environs of the village of Zamosc near Bydgoszcz.

"The situation does not threaten the safety of residents. The location of the discovery is being investigated" by Polish officers and military police, the ministry said.

The area where the object was found is hundreds of kilometres from Poland's borders with Ukraine, Belarus and the Russian exclave of Kaliningrad.

RMF said that at first authorities thought the object was a drone, but later determined that it could be the remains of missile. According to RMF it was not known whether it had exploded.

The military police, regional police, the mayor of the village of Zamosc, the Gdansk prosecutor's office and a government spokesman all declined to comment further. (Reporting by Pawel Florkiewicz and Anna Koper, writing by Alan Charlish; Editing by Lincoln Feast, Bernadette Baum, Peter Graff)