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 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 defence, as well as a warship, its defence ministry said.
Danish Prime Minister Mette Frederiksen said, "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 said it will contribute three Rafale fighter jets, and Germany has pledged four Eurofighters.
The UK is "fully committed" to help strengthen the Eastern Sentry operation, the Ministry of Defence said in a statement, adding that it will provide more details soon.
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