Russia’s top security agency says that it has arrested several suspects accused of involvement in an alleged Ukrainian plot to assassinate senior military officers.

Russia has arrested four people on suspicion of plotting to assassinate senior defence officials and their families on Ukraine’s orders, the Kremlin’s Federal Security Service (FSB) said.

A Ukrainian official said Kyiv was responsible for the assassination in Moscow of Gen. Igor Kirillov, the chief of Russia’s radioactive, chemical and biological defense forces.

Igor Kirillov, who was named the head of Russia’s nuclear defense forces in April 2017, was under sanctions from several countries including the U.K. and Canada for his role in Ukraine.