BEIJING, Sept 1 (Reuters) - Activity in China's vast manufacturing sector contracted at its fastest pace in three years in August, an official survey showed on Tuesday, reinforcing concerns over the health of the world's second-largest economy despite a flurry of government support measures.

The official Purchasing Managers' Index (PMI) fell to 49.7 in August from the previous month's reading of 50.0, in line with expectations of analysts polled by Reuters.

A reading over 50 points signals an expansion in activity while one below that mark indicates an contraction on a monthly basis.

Beijing has set an official economic growth target of 7 percent for this year, down from 7.4 percent in 2014 and the slowest pace in a quarter of a century.

But some economists believe real growth levels may already be much lower, with cooling domestic and export demand, factory overcapacity and high local government debt levels depressing investment.

A summer stock market crash and Beijing's surprise devaluation of its currency in mid-August have heightened concerns about growing risks to the economy, roiling global financial markets in recent weeks.

(Reporting by Kevin Yao and Winni Zhou; Editing by Kim Coghill)