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 said 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 said.
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 committed three Rafale jets, and Germany has pledged four Eurofighters. The UK also voiced its commitment to strengthen the Eastern Sentry operation.
On Friday, European countries and the US backed Poland during an urgent UN Security Council session discussing the incursion. Acting US Ambassador to the UN Dorothy Shea reaffirmed that the United States stands by NATO allies against airspace violations.
Poland's Secretary of State Marcin Bosacki presented evidence of the drone incursions at the UN, insisting that it was not a mistake. Conversely, Russia maintained it had not targeted Poland, claiming its drones could not have physically reached Polish territory.
Amid ongoing military drills near the borders with Poland, Russia and Belarus dismissed claims that their actions posed any threat to neighboring countries, exacerbating concerns in the region.
