(Bloomberg) — A Russian missile briefly entered Poland’s airspace on Friday after Moscow attacked neighboring Ukraine with the biggest aerial barrage of the 22-month invasion, army commanders said in Warsaw.

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The missile flew over the border at around 7 a.m. and traveled about 40 kilometers (25 miles) deep into the Polish airspace before returning to Ukraine, General Wieslaw Kukula told a news conference.

Poland, a NATO and European Union member, has been on alert ever since a stray rocket killed two people in a village in eastern Poland more than a year ago, following one of Russia’s previous attacks on Ukraine. Another missile was discovered in the woods in central Poland earlier this year, five months after it entered the country’s airspace.

NATO Secretary Jens Stoltenberg said in a post on X social media platform that the military alliance was “monitoring the situation” and “remains alert” following his phone call with President Andrzej Duda. US National Security Adviser Jake Sullivan pledged “technical assistance as needed,” according to a readout of a conversation with his Polish counterpart, Jacek Siewiera.

Russia’s intense aerial attack on Friday hit mostly civilian targets in Ukraine, killing at least 26 people and wounding over 100 others days after Moscow blamed Kyiv for a strike on a military ship in Crimea.

Read more: Russia Slams Ukraine With Record Missile Barrage, Killing 26

Poland’s military monitored the missile and is currently conducting an on-the-ground search to ensure the missile or its components didn’t fall on the ground, Kukula said.

Grazyna Ignaczak-Bandych, head of Polish President’s Chancellery, told reporters that there’s currently no threat from the aerial attacks following a meeting of the country’s National Security Bureau.

–With assistance from Justin Sink and Alisa Parenti.

(Updates with NATO and US response from the fourth paragraph.)

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