An unexplained item that entered Polish airspace from Belarus was most likely an observation balloon, Poland’s defense ministry tweeted on Saturday.

NATO member Poland is on high alert for violations of its airspace as a result of the conflict in neighboring Ukraine, following events in November and December in which missiles landed on its territory.

According to Captain Ewa Zlotnicka, press secretary for the Armed Forces Operational Command, the object entered Polish airspace on Friday evening.

The ministry said radar contact with it was lost near Rypin, a town in north-central Poland. A Territorial Defence Force spokesman said a search for it was under way.

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 defense missile, and the discovery of the remains of the missile in northern Poland in April, national security is a key issue in Poland ahead of elections later this year.