Business activity across Japan's private sector weakened in September, with the final au Jibun Bank Japan Composite Output Index, compiled by S&P Global, coming in at 52.3 from 53.5 in August, according to data released on Monday.

The reading, which tracks combined output across the manufacturing and service sectors, marks the 18th straight month of expansion for Japan's business sector, but the slowest since May.

A print above 50 indicates an overall expansion in private sector activity, while a figure below signals a contraction.

The monthly momentum reflected a synchronized performance across the economy. The manufacturing PMI fell to 54.1 from 54.9 the previous month, while the final services PMI edged down to 51.3 from a five-month high of 52.5.

New business across both manufacturing and services sectors rose at a weaker pace in September despite a stronger rise in new exports, driven by the manufacturing sector.

Overall employment grew at the fastest pace since February.