June 21 (Reuters) - Salad chain sweetgreen Inc said on Monday it had confidentially filed for an initial public offering in the United States. The company was valued at $1.8 billion after a funding round earlier this year, according to media reports. Sweetgreen did not reveal more details about the size of the proposed IPO. (Reporting by Niket Nishant in Bengaluru; Editing by Shinjini Ganguli) Our Standards: The Thomson Reuters Trust Principles.