An unidentified object that entered Polish airspace from the direction of Belarus was probably an observation balloon, the defence ministry said on Saturday on Twitter.
NATO-member Poland is on alert for infractions of its airspace as war rages in neighbouring Ukraine after incidents in November and December in which missiles landed on its territory.
"The Air Operations Center noted the appearance in Polish airspace of an object that flew in from the direction of Belarus," the defence ministry said on Twitter.
"It is probably an observation balloon," it added. "Radar contact was lost near Rypin", a town in north-central Poland.
A spokesperson for Poland's Territorial Defence Force said a search for the object was underway.
Russian and Belarusian authorities were not immediately available for comment.
After the deaths in November of two Poles hit by what Warsaw concluded was a misfired Ukrainian air defence missile, and the discovery of a military object in northern Poland in April, national security is a key issue in Poland ahead of elections later this year.
