In a move that could tip the balance of the Ukrainian conflict in favor of the Russian-backed separatists, Russia is sending a unit of advanced interceptor fighters to an airbase close to the Ukrainian border. Operating from there, the Mig-31 fighter jets would be able to survey airspace well inside Ukraine without actually flying over the country, and alert separatists on the ground of any movements by Ukrainian airplanes.

The National Security and Defense Council of Ukraine tweeted on Wednesday that Russia was moving “Mig-31 fighter jets” to the border.