Poland’s armed forces chief believes a Russian missile entered Poland for almost three minutes and then turned back into Ukrainian airspace.
Gen Wieslaw Kukula said the missile travelled about 40km (25 miles) into Polish airspace early on Friday.
The alert coincided with what Ukraine has called Russia’s biggest day of air strikes since its war began.
Poland’s UN representative said Warsaw believes the incident was linked to the massive bombardment.
“We clearly see it as an element of the situation created by the latest wave of Russian attacks,” Krzysztof Szczerski told the UN Security Council.
President Andrzej Duda convened an emergency security meeting after the object was picked up on radar.
About 200 police officers have been conducting a search of the area where the object was detected in case the missile landed on Polish territory.
Poland is a member of the Nato alliance, and Polish and Allied aircraft were scrambled in response to the incident at around 07:00 (06:00 GMT) on Friday. There have been no reports of an explosion.
Operational Command spokesman Lt Col Jacek Goryszewski said an unidentified object had entered Poland from Ukraine near the town of Zamosc, in the Lublin region of south-eastern Poland, not far from the border.
At least 30 people were killed in the attacks which targeted Lviv, the closest Ukrainian city to Lublin region, as well as Dnipro, Kyiv and other cities.
The Polish military was tracking the Russian missile strikes during the night. Nato Secretary General Jens Stoltenberg said the alliance stood in solidarity with Poland and “remains vigilant”.
An unconfirmed report said a search was taking place near the town of Hrubieszow.
The Polish government has not ruled out Russian provocation. “We need to check whether this wasn’t a provocation or a test of our reaction. We should check both scenarios carefully,” Deputy Defence Minister Stanislaw Wziatek told TVN24.
Polish military expert Cmdr Maksymilian Dura told TVN24 it was premature to conclude it was a Russian missile because it had not been found, and just because contact had been lost it could not be certain that it had left Polish airspace.
Krzysztof Komorski, the president of the Lublin Voivodship [equivalent to a province or region] wrote on social media: “Please be calm and patient, the services are working.”
Since Russia’s invasion and war against Ukraine three missiles have entered Poland.
In November 2022, two Polish farmers were killed by a missile that landed in the village of Przewodow near the Ukrainian border. It was believed to have been fired by Ukrainian air defence forces to repel a Russian missile attack.
In a harmless but more embarrassing incident from December last year, an object believed to have been an unarmed Russian Kh-55 cruise missile was fired from Belarus and crossed around 500km of Polish territory before landing in a forest.
The object, which was detected by Polish air defences at the time, was only found in April this year by a passer-by not far from the city of Bydgoszcz in central Poland.
Another unidentified object that entered Polish airspace this year from the direction of Belarus was probably an observation balloon. Radar contact with it was lost near Rypin in central Poland.
Both sides of Poland’s political arena have seized on the latest event to score points.
Poland’s new defence minister, Wladyslaw Kosiniak-Kamysz, stressed in a post on X, formerly known as Twitter, that the security forces “acted immediately” to the latest incident, reassuring Poles, “the state is acting!”.
His predecessor, Mariusz Blaszczak, from the right-wing Law and Justice-led government that lost power in an election in October and served as defence minister during the two earlier events, responded: “We don’t know what fell in the area of Tomaszow Lubelski. We don’t know if anyone was hurt. We don’t know why the anti-aircraft defence systems didn’t work. Is the state acting?”