SEOUL, Sept 30 (Reuters) - South Korea’s factory output in August unexpectedly declined from a month earlier, but continued to log growth on a year-on-year basis for the 10th straight month helped by strong demand for semiconductors. Industrial production last month fell by a seasonally adjusted 0.7%, missing a 0.5% gain forecast in a Reuters survey and following 0.2% growth in July. Output expanded 9.6% on an annual basis, better than 8.2% growth forecast in the poll and a 7.7% expansion in July. Reporting by Joori Roh; Editing by Jacqueline Wong Our Standards: The Thomson Reuters Trust Principles.