May 22 (Reuters) - Singapore Telecommunications STEL.SI reported a 9% rise in full-year underlying net profit on Thursday, boosted by strong performances from unit Optus and regional associates Airtel and AIS, while announcing a new S$2 billion ($1.55 billion) share buyback.

Southeast Asia's largest telecom firm said its underlying net profit for the year ended March 31 was S$2.47 billion ($1.91 billion), compared with S$2.26 billion a year earlier.

That was slightly lower than a Visible Alpha consensus estimate of S$2.56 billion.

($1 = 1.2899 Singapore dollars)

(Reporting by Himanshi Akhand and Rajasik Mukherjee in Bengaluru; Editing by Vijay Kishore and Mohammed Safi Shamsi)

((Himanshi.Akhand@thomsonreuters.com; Rajasik.Mukherjee@thomsonreuters.com;))