MOSCOW: Ukraine's dramatic drone attack on Russian bombers over the weekend has increased the risk of escalation to "unacceptable" levels, said US envoy Keith Kellogg. "The risk levels are going way up,

U.S. 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.

President Putin reportedly vowed to respond to Ukraine's surprise attack on its strategic bombers. According to Russia's nuclear doctrine, the attack could technically prompt a nuclear retaliation.