Online news site Ukrainska Pravda reports that the assailant had used a pistol and had shot the SBU officer five times, citing unnamed sources.
The apparent assassination follows what Ukraine described as the largest Russian aerial attack on Tuesday, when 728 drones and 13 cruise or ballistic missiles struck cities across the country.
Overnight into Thursday, a Russian attack on the Ukrainian capital killed at least two people and injured 16 others.
The strikes – which hit eight districts across the city – involved 18 missiles and 400 drones, officials said. Russia has been repeatedly accused of targeting civilian areas.
Meanwhile, fighting on the front line continues, with Russian forces slowly making advances in western Ukraine and retaking control of the part of Russia’s Kursk region that Ukrainian forces seized in a surprise offensive last summer.
Russia currently controls around a fifth of Ukrainian territory, including the southern Crimean peninsula it annexed in 2014.
Efforts to negotiate a ceasefire in the more than three-year-long war have faltered, with US President Donald Trump becoming increasingly impatient with his Russian counterpart, Vladimir Putin.