British fighter jets have flown their first mission over Poland as part of operations to protect NATO's eastern flank.
It followed repeated violations of NATO airspace by Russian drones and planes in recent weeks.
UK Defence Minister John Healey said the Eastern Sentry mission had sent "a clear signal" that NATO airspace would be defended.
"NATO airspace will be defended," Mr Healey said in a statement.
Two Royal Air Force Typhoons took off from a British military base in England on Friday night to patrol Polish skies and defend against aerial threats from Russia, including drones, the statement said, adding that they returned safely to the UK early Saturday morning.
The British government said it was a response to the "most significant violation" of NATO airspace by Russian President Putin to date since his illegal full-scale war in Ukraine.
The Russian incursions sparked calls for a tougher response, with Czech President Petr Pavel saying Russian aircraft entering European airspace should be shot down.
UK remains 'agile and ready'
Air Chief Marshal Harv Smyth said the British jets joined allies along NATO's eastern flank.
"We remain agile, integrated, and ready to project airpower at range," he said.
The UK government has promised to boost defence spending in a signal to US President Donald Trump that Britain can help boost Europe's security.
Mr Trump has criticised European nations for not spending enough on defence and relying instead on the United States.
The UK mission over Poland also comes amid heightened tensions elsewhere in Europe, with NATO-member Estonia saying on Friday three Russian military jets violated its airspace for 12 minutes in an "unprecedentedly brazen" incursion.
Russian jets over Estonia ignored signals from NATO pilots, officials say
"[Russia's] latest reckless and dangerous activity is the third violation of NATO airspace in recent days," Mr Healey said.
Russian pilots ignored signals from Italian jets responding from NATO's Baltic Air Policing Mission when they violated Estonian airspace, a senior Estonian military official said on Saturday.
Russia's Defence Ministry denied its jets violated Estonian airspace, saying they flew over neutral waters.
Estonian officials dismissed the denial, saying the violation was confirmed by radar and visual contact and suggested it could be a tactic to draw Western resources away from Ukraine.
The Russian MiG-31 fighters entered the area of Vaindloo, a small island located in the Gulf of Finland in the Baltic Sea, the Estonian military said.
A ministry statement said it was the fourth airspace violation by Russia this year.
It still "needs to be confirmed", if the border violation was deliberate or not, said Colonel Ants Kiviselg, the commander of Estonia's Military Intelligence Center.
Regardless, he said, the Russian jets "must have known that they are in [Estonian] airspace".
The Russian pilots did not pose a "military threat", he said.
The Russian pilots acknowledged the Italian pilots flying F-35 fighter jets, but they ignored following the signals, which is partly why they were in Estonian airspace for so long, he added.
"Why they didn't do it, that's a question for the Russian pilots," Colonel Kiviselg said.
ABC/wires
