Yvette Cooper issued a stern warning to "warmongering" Vladimir Putin that Britain will confront Russian jets that invade the airspace of NATO allies.
It comes after a string of incursions by Russian drones and planes, most recently into Estonian airspace.
Speaking at an emergency meeting of the UN Security Council, Ms Cooper sent a message to Putin: "Your reckless actions risk a direct armed confrontation between NATO and Russia.
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“Our alliance is defensive but be under no illusion, we stand ready to defend NATO's skies and NATO's territory.
“We are vigilant, we are resolute, and if we need to confront planes that are operating in NATO airspace without permission, we will do so."
She added: "The United Kingdom stands by our friends."
Ukraine's President Volodymyr Zelensky anticipated “a very intense week” of diplomacy at UNGA in New York, where the Security Council was expected discuss the more than three-year war.
International concerns are mounting that the fighting could spread beyond Ukraine ’s borders as European countries rebuked Russia for incursions into NATO airspace.
The incidents have included Russian drones landing on Polish soil and Russian fighter aircraft entering Estonian airspace.
On September 19, three Russian jets entered Estonian airspace without permission, remaining there for 12 minutes and being intercepted by Italian aircraft operating as part of Nato's Eastern Sentry mission.
Ms Cooper added: “We are committed to increasing the pressure on Russia until it ends its illegal war in Ukraine, ceases its reckless acts and respects the sovereignty of other states.
“Russia's imperialist warmongering is a threat to international peace and security. It is a threat to the values and principles that underpin this organisation.
“This organisation that has done so much to support stability and prosperity for the last 80 years. We must counter it for the sake of us all.”
Polish Prime Minister Donald Tusk said his country was the closest to "open conflict" it had been since the Second World War, while the EU's high representative on foreign affairs warned Russia was "testing the West's resolve".
In response to the done incursion over Poland, the UK announced that RAF Typhoon jets would also take part in Eastern Sentry, flying their first mission over Poland late on September 19.
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