Russia's top security agency says it has arrested four Russians accused of making preparations to kill senior Defense Ministry officials on Ukraine's orders.

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.

Lieutenant General Igor Kirillov, head of Russia’s Nuclear, Biological, Chemical Defence Forces (NBC), was killed in an explosion in Moscow in an assassination claimed by Ukraine

The arrest announcement follows the death of Russia's Lt. Gen Igor Kirillov, who was killed on Dec. 17 by a bomb hidden on an electric scooter parked outside his apartment building.

According to the Federal Security Service (FSB), four Russians were involved in an alleged Ukrainian plot to assassinate senior military officers.

The bombs were disguised as an electric scooter, a document folder and a magnetized portable charger, Russia's Federal Security Service said.

Russia's security service said four Russian citizens were recruited by Ukraine to carry out the assassinations

The Federal Security Service of Russia said a series of assassination attempts had been foiled. Four Russian citizens have been arrested.