Putin hails successful launch, saying Sarmat will make countries 'think twice' about aggressive intentions against Moscow

Moscow issues warning after alleged Ukrainian helicopter strikes on Russian border towns, which Ukraine has denied; claims to kill 'up to 30 Polish mercenaries'

Scientists killed at secretive nuclear research facility were believed to be developing Burevestnik nuclear-powered cruise missile

Vessel said to be top-secret nuclear-powered submersible used for sensitive missions at great ocean depths