Ukraine's intelligence directorate said on Sunday that an explosion in the Russian-occupied town of Melitopol killed at least three Russian servicemen in an attack it described as an "act of revenge" by local underground groups. The blast occurred during a meeting on Saturday of Russian officers in the town, a hub of Russian forces in the south, the directorate within the defence ministry said in a statement. There was no mention of the incident from Russian officials. Reuters was unable to verify the Ukrainian statement.