It's becoming almost par for the course these days, even though the Cold War ended long ago: NATO aircraft dispatched to intercept Russian military aircraft encroaching on the edges of their countries' airspace.

But Thursday's flight by a pair of Russian strategic bombers was remarkable even by the standards of this new conflict between Moscow and the West. Two Tu-95 "Bear" bombers – with their transponders off to foil radar – flew through the busy air corridor over the English Channel. The appearance of the warplanes forced Britain's Royal Air Force to scramble its own jets and caused civilian air traffic to reroute.

While the world is becoming used to Russian sabre-rattling amid the showdown over Ukraine, this flight of the Bears was a dangerous raising of the stakes, and came on the same day that the man who ended the first Cold War – former Soviet leader Mikhail Gorbachev – warned that the latest confrontation between Russia and the West had the potential to devolve into a "hot" conflict.

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"A cold war is already being waged openly. What will happen next?" Mr. Gorbachev said in an interview with the Interfax news service. He went on to accuse the United States of "triumphalism" since the Cold War ended with the fall of the Soviet Union in 1991, and said that attitude had led to today's confrontation. "Unfortunately, I can no longer say firmly that the cold war won't turn into a hot one."

Mr. Gorbachev spoke out in a week that has seen a return to heavy fighting in southeastern Ukraine, where a Russian-backed insurgency has established a de facto mini-state around the cities of Donetsk and Lugansk.

Britain called the Russian bomber flights over the Channel "a serious escalation" and summoned Russian Ambassador Alexander Yakovenko to give an explanation. "It was very dangerous. Civil aircraft flying to the U.K. had to be rerouted," an unnamed British government source told Reuters. "The Russians were flying with their transponders turned off so could only be seen on military radar. They haven't flown this far south before."

The Kremlin-run Sputnik news service reported that the two Tupolevs "did not breach international regulations or any nation's borders."

NATO says it intercepted more than 100 Russian military aircraft last year, a threefold increase from 2013, as tensions soared following a pro-Western revolution in Kiev. Moscow subsequently annexed the Crimean peninsula from its neighbour and lent support to the uprisings in Donetsk and Lugansk.

Swedish authorities say a Russian military jet nearly collided with a commercial passenger airplane near its airspace in December. Russia has denied that its warplanes were ever dangerously close to passenger aircraft.

The NATO military alliance has accused the Russian military of direct involvement as separatist forces have broken through old front lines in Ukraine to capture Donetsk's international airport and lay siege to the Ukrainian government-controlled cities of Mariupol and Debaltsevo. Both cities are considered strategically important, with Mariupol being the region's biggest port, and Debaltsevo astride a key road between Donetsk and Lugansk.

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Ukrainian President Petro Poroshenko also told the World Economic Forum last week that 9,000 Russian troops were operating on his country's soil. Ukraine's parliament, the Verkhovna Rada, has subsequently sought to designate the Donetsk and Lugansk rebels as "terrorists," hoping that will draw more Western military support to the Ukrainian side.

So far, however, Canada, the United States and the EU have offered only non-military aid to Ukraine's outgunned military.

Russian President Vladimir Putin has denied the accusations of direct Russian involvement, though Kremlin-controlled media have acknowledged the presence of Russian "volunteers" in eastern Ukraine. This week, Mr. Putin referred to the Ukrainian army as a NATO "foreign legion" that acted in the interests of Washington instead of Kiev.

Other Russian officials have hardened their rhetoric over Ukraine in recent days. On Thursday, Russia's representative to the Organization for Security and Co-operation in Europe warned of a "big catastrophe" if the West continued to support Mr. Poroshenko's government.

Mr. Gorbachev, who in the past has made headlines with his criticisms of Mr. Putin, said the conflict between Russia and the West was approaching a point of no return in Ukraine. "If our country gets involved [in the fighting], it could spark a fire so strong the whole world won't be able to put it out."

European Union foreign ministers, meanwhile, broke a deadlock on Thursday by getting Greece's new left-wing government – seen as friendly to Moscow – to sign on to an extension of the economic sanctions against Russian businesses and individuals that were due to expire in February. The sanctions will now remain in place until September, and the EU also agreed on a new list of targets that will be added to the sanctions list on Feb. 9, unless there are signs of progress toward peace in Ukraine.

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The United States said Thursday that it was also considering new economic measures to punish Russia. State Department spokeswoman Jan Psaki said the EU's decision was "just a further sign that the actions of the last several days and weeks are absolutely unacceptable and that there will be new consequences put in place."