NATO is to bolster Europe's eastern flank, including the use of UK military resources, after Russia's "reckless and unacceptable" violation of Polish airspace.
NATO Secretary-General Mark Rutte announced operation 'Eastern Sentry' on Friday, involving the deployment of equipment on the border with Belarus, Russia and Ukraine to deter potential Russian aggression.
Poland shot down Russian drones which flew over the country on Wednesday, something the military alliance has portrayed as an attempt by Moscow to test NATO's military response.
It underlines long-held concerns about the potential expansion of Russia's three-year war in Ukraine.
Russia said its drones went astray because they were jammed, but European leaders believe the incursions were a deliberate provocation by Russia.
"It's reckless and unacceptable. We can't have Russian drones entering allied airspace," Mr Rutte told a news conference.
He added that allies, including the UK, France, Germany and Denmark, have so far committed to the mission with others set to join.
In a statement, the UK's Ministry of Defence said: "The UK is fully committed to playing our part in NATO's Eastern Sentry following the reckless and dangerous airspace violations by Russia". It added that the details of the UK's contribution would be announced soon.
Yvette Cooper visited Ukraine on Friday, her first visit to the country since becoming foreign secretary after a cabinet shake-up by Sir Keir Starmer.
Ms Cooper described her trip, which included a meeting with the country's president, Volodymyr Zelenskyy, as a demonstration of solidarity with Ukraine.
"The UK will not stand idly by as Putin continues his barbaric invasion of Ukraine," Ms Cooper said, noting what she said was the Russian president's "complete disregard for sovereignty" by sending drones into NATO airspace.
Prince Harry also made a surprise visit to Kyiv on Friday, where he met with wounded service members.
NATO already has substantial forces in eastern Europe, including thousands of troops, but the alliance did not explain how many additional forces would be involved in the new operation.
Speaking at the alliance's Brussels headquarters, US General Alexus Grynkewich told reporters the additional resources will enable the alliance to "plug gaps in the line" and concentrate forces wherever they're needed while improving communications across NATO's entire eastern flank.
NATO detailed a modest number of additional military assets - including two F-16 fighter jets and a frigate from Denmark, three Rafale fighter jets from France and four Eurofighter jets from Germany.
The announcement also coincided with a joint military exercise involving thousands of Russian and Belarusian troops, which began on Friday.
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Meanwhile, new measures were announced by the UK against Russia on Friday.
They included bans on 70 vessels the UK says are part of Russia's "shadow fleet" that transports Russian oil in defiance of sanctions.
Some 30 individuals and companies - including Chinese and Turkey-based firms - were also sanctioned for their part in supplying Russia with electronics, chemicals, explosives and other weapons components.
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Donald Tusk, the Polish prime minister, dismissed Donald Trump's suggestion that the drone incursion into Poland may have been "a mistake".
"We would also wish that the drone attack on Poland was a mistake," Mr Tusk wrote on X. "But it wasn't. And we know it."
US-led efforts to steer Moscow and Kyiv toward a peace settlement have so far failed to progress.
Kremlin spokesperson Dmitry Peskov said talks to end the war were currently on "pause" even though channels of communication remain open.
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