BEIJING, Sept 1 (Reuters) - Activity in China's manufacturing sector unexpectedly expanded in August, though growth was modest, an official survey showed on Thursday.

The official Purchasing Managers' Index (PMI) rose to 50.4 in August, compared with the previous month's 49.9 and above the 50-point mark that separates growth from contraction on a monthly basis.

Analysts polled by Reuters had predicted a reading of 49.9.

(Reporting by Yawen Chen and Elias Glenn; Editing by Kim Coghill)