President Donald Trump's Ukraine envoy said the risk of escalation from the war in Ukraine was "going way up" after Ukrainian forces used drones to strike nuclear-capable bombers at several airbases deep inside Russia.
Ukraine said it attacked airfields in Siberia and Russia's far north over the weekend, striking targets up to 4,300 km (2,670 miles) from the front lines of the conflict.
"I'm telling you, the risk levels are going way up - I mean, what happened this weekend," Trump's envoy, Keith Kellogg, told Fox News.
"People have to understand in the national security space: when you attack an opponent's part of their national survival system, which is their triad, the nuclear triad, that means your risk level goes up because you don't know what the other side is going to do. You're not sure."
Russia and the United States together hold about 88% of all nuclear weapons.
Each power has three main ways of attacking with nuclear warheads, known as the nuclear triad: strategic bombers, land-launched intercontinental ballistic missiles and submarine-launched ballistic missiles.
Kellogg said the damage to the Russian bombers at the weekend was less important than the psychological impact on Russia and that he was particularly concerned by unconfirmed reports of a Ukrainian attack on a naval base in northern Russia.
White House spokeswoman Karoline Leavitt said on Tuesday that Trump had not been informed in advance of Ukraine's drone attacks on Russia's bombers.
Russia and Ukraine held talks in Istanbul on Monday but made little headway towards ending the war that has raged since Moscow sent tens of thousands of troops into Ukraine more than three years ago.
Kellogg said Ukraine had come up with a "very reasonable position" but Russia had come with a "very maximalist position," and that the aim now was to "try to bridge that."
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