Asia's factory activity weakened in September as soft Chinese demand and global economic uncertainty pointed to a challenging outlook, private-sector surveys showed, keeping policymakers under pressure to shore up fragile growth. The region's manufacturers may get some relief in coming months from aggressive stimulus unveiled by Chinese authorities over the past week, including a lowering of interest rates and injection of liquidity into the banking system. Factory activity in Japan shrank in September and expanded at a slower pace in Taiwan, purchasing managers' index (PMI) surveys showed on Tuesday, highlighting the toll soft global demand was taking on Asian exporters.