Several reports say that explosions on Sunday rocked the Russian city of Belgorod, just north of the Ukrainian border.
A Russian official quoted in the reports says that three people were killed when the blasts hit several residential buildings.
This could be a tit-for-tat move by Ukraine, since it comes two days after Russia targeted the Ukrainian port city of Odesa with missiles.
One of the reports, in The New York Times, says that images posted by social media show smoke rising from parts of the city, with at least one house on fire.
Ukraine has targeted Belgorod before, usually hitting gas tanks and other non-residential targets.