More NATO countries will move troops and fighter jets eastwards after more than a dozen drones entered Polish airspace on Wednesday.
Denmark, France, and Germany have joined a new mission to bolster the military alliance's eastern flank. Other NATO allies are expected to take part later.
Tensions have been high across Europe since Poland accused Russia of an unprecedented incursion. Some of the 19 drones that entered Polish territory were shot down, while others crashed into fields and even a house in eastern Poland.
Warsaw said the incursion was deliberate, but Moscow downplayed the incident, saying it had no plans to target facilities in Poland.
Drones and missiles have occasionally veered into countries bordering Ukraine, including Poland, during Russia's full-scale invasion - but this was the most serious incident of its kind since the war began in February 2022.
Polish Prime Minister Donald Tusk stated it was the closest we have been to open conflict since World War Two.
Denmark will contribute two F-16 fighter jets to support Poland's air defense, as well as a warship, its defense ministry announced. Danish Prime Minister Mette Frederiksen stated, We must not be naive. Putin will stop at nothing, and he is testing us. Therefore, it is crucial... Denmark is contributing to this.
France has already pledged three Rafale fighter jets, and Germany has promised four Eurofighters. The UK also reassured its commitment to strengthening the Eastern Sentry operation.
On Friday, European countries and the US backed Poland during an urgent UN Security Council session discussing the incursion. The acting US Ambassador to the UN, Dorothy Shea, expressed strong support for NATO allies amid alarming airspace violations, asserting that we will defend every inch of NATO territory.
After presenting evidence of the drone incursions at the UN, Poland maintained that these actions were not coincidental. Meanwhile, Russia dismissed the accusations, claiming its drones did not have the range to reach Polish territory.
As the case unfolds, greater military coordination among NATO allies is likely as they adapt to the shifting dynamics posed by Russia's aggressive incursions.