China's manufacturing activity unexpectedly fell in May, keeping alive calls for fresh stimulus as a protracted property crisis in the world's second-largest economy continues to weigh on business, consumer and investor confidence.

The official manufacturing purchasing managers' index (PMI) dropped to 49.5 in May from 50.4 in April, the National Bureau of Statistics (NBS) said on Friday, below the 50 mark separating growth from contraction and missing analysts' forecast of 50.4.

The disappointing number adds to a series of recent indicators showing the $18.6 trillion economy is struggling to get back on its feet, eroding earlier optimism seen after better-than-expected output and trade data.