Have deleted this answerThis is a well-known fact in Banach space theory (even locally convex space theory in a suitable form). It was intended to be helpful butSee, alasfor example, led to the accusation that I am a trollclassical monographs of Köthe or Schaefer. The direction you are looking for follows from the Alaoglu theorem (for the version involving compactness). For the variants, see the Eberlein-Smulian theorem, e.g., in Grothendieck's text.