Poland said it rapidly mobilized military aircraft early Sunday to secure its airspace after Russia launched airstrikes on Ukraine’s Lviv region near the Polish border.

“Polish and allied aircraft are intensively operating in our airspace, while ground-based air defense and radar reconnaissance systems have reached the highest state of readiness,” said the Operational Command of the Polish Armed Forces.

Ukrainian officials said the Russian strikes killed and at least four people in the Lyiv region and one person in the southeastern city of Zaporizhzhia.

The mayor of Lviv said public transport routes were not operating due to a “massive enemy attack,” which had also caused a partial electricity outage in the region.

“Russia targeted residential areas with drones and aerial bombs. Across all affected areas, residential buildings and critical infrastructure were damaged,” Ukraine’s Prime Minister Yulia Svyrydenko said on X.

It was just the latest instance of Poland scrambling fighter planes to protect its airspace amid Moscow’s attacks on Ukraine. In early September, Poland shot down Russian drones that forced the closure of the Warsaw airport as Russian President Vladimir Putin appeared to be testing NATO’s defense capabilities.

Since then, eastern-flank members of NATO have been on high alert. Denmark had to close its airspace last week after drone activity was detected around Skrydstrup Air Base.