Russia has accused Ukraine of carrying out a number of high-profile attacks against prominent Russians since Moscow invaded its neighbor nearly four years ago.

Ukraine’s Security Service has claimed responsibility for killing Lt. General Igor Kirillov, head of the military’s nuclear, biological and chemical protection forces, in a bombing in Moscow.

A 64-year-old man has been arrested in a southern Russian city for the shooting death of a former submarine commander who is alleged by some in Ukraine to have killed civilians in a long-range missile strike last year.