A Russian missile may have struck the territory of Poland, a NATO member, during an overnight assault on neighboring Ukraine, according to Polish authorities, an event that risked bringing Russia’s war closer to direct confrontation with the Western military alliance.

“So far, everything indicates that we are dealing with a Russian Kh-101 ballistic missile,” Prime Minister Donald Tusk of Poland told an emergency meeting on Thursday, according to the Polish national news agency, PAP. Mr. Tusk appeared to have incorrectly identified the missile as ballistic. The Kh-101 is a cruise missile.

He said no one was injured where the missile fell, which was about 50 kilometers, or 31 miles, south of where Mr. Tusk and President Volodymyr Zelensky of Ukraine met on Wednesday, in the city of Lublin.

“We were ready to shoot down this missile if it continued its flight,” Mr. Tusk said.

Ukraine’s foreign minister, Andrii Sybiha, said on social media that a Russian Kh-101 cruise missile had crossed into Poland, but did not specify whether the weapon had hit the country.

Representatives for NATO did not immediately respond to requests for comment.

Earlier in the day, Polish authorities had said that an “unidentified object” had crossed into the country’s airspace overnight and that police officers later found a 10-meter crater on a field in southeastern Poland, along with debris from the object.

Russian missiles have briefly entered Polish airspace during previous attacks targeting western regions of Ukraine, which border Poland, but the missiles have never hit Poland itself. Any confirmed strike on a NATO member could sharply escalate tensions between the military alliance and Russia.

In a statement, Poland’s military said it had been monitoring at least a dozen Russian missiles flying over western Ukraine overnight, including what was described as “an object” that approached the Polish border around 3:29 a.m. local time but then changed its direction and headed east.

Around 10 minutes later, the military statement said, an unidentified object was detected in Polish airspace. Before an F-16 fighter jet that was scrambled could intercept it, however, the object disappeared from radar. Polish authorities sent a helicopter crew to investigate and found what the military statement called “the probable crash site of the object” near the village of Tarnawa-Kolonia, in the southeastern Lublin province.

Poland’s defense minister, Wladyslaw Kosiniak-Kamysz, said on social media Thursday morning that it remained unclear what the object was or from where it originated. He called the incident “an extremely tough night for our air defense, observation and threat detection systems.”

“Clarification is now underway regarding what type of object fell in the vicinity of the town of Tarnawa-Kolonia,” he said.

Anatol Magdziarz contributed reporting from Warsaw.