An incident involving a US spy drone and a Russian fighter jet is a signal that Russian President Vladimir Putin is ready to expand the Ukraine-Russia conflict zone, a senior Ukrainian security official said on Wednesday. The US military said on Tuesday a Russian fighter plane had clipped the propeller of a spy drone and made it crash into the Black Sea. Moscow denied the aircraft had collided and said the drone had crashed after "sharp maneuvers". The Ukrainian presidency and government did not immediately offer any official comment on the incident but Oleksiy Danilov, Secretary of Ukraine's National Security and Defense Council, voiced concern on Twitter. "The incident with the American MQ-9 Reaper UAV, provoked by Russia in the Black Sea, is Putin's signal of readiness to expand the conflict zone with the involvement of other parties," he wrote. He said Russia had a tactic of going all-out with a "constant raising of the stakes" in the hope of turning round what he described as the "conditions of a strategic defeat" for Russia in its year-old war in Ukraine.