SHANGHAI/BEIJING, May 25 (Reuters) - China’s major state-owned banks were seen buying U.S. dollars at around 6.4 yuan per dollar level on Tuesday afternoon, four sources with knowledge of the matter said, in a move viewed as an effort to curb fast yuan appreciation to breach the key level. (Reporting by Winni Zhou, Jindong Zhang, Xiao Han, Rong Ma and Andrew Galbraith; Editing by Andrew Heavens)