The official purchasing managers index (PMI) hit 50.8 in the month, up from 50.4 in April, the National Bureau of Statistics said. Any reading above 50 indicates growth.
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The country's economy grew by 7.4% in the first three months of the year, down from 7.7% growth in the previous quarter.
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Last month, another PMI survey by HSBC gave a preliminary reading for May of 49.7, also a five-month high. This survey focuses on smaller companies in the private sector, while the official PMI survey is weighted more towards bigger, state-owned enterprises.
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"The improvement of both PMIs suggests that economic activities have stabilized somewhat due to the recent pro-growth policies," said ANZ bank.
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These policies have included extending tax breaks for small and medium-sized companies, and ramping up spending on China's railway infrastructure.
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Last week, the government also signaled it would cut reserve requirements for banks in a bid to stimulate lending to certain sectors.
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At the same time as announcing the measures, the State Council described the economy as "stable", but added that "downward pressure is still relatively large".
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Earlier this year, Premier Li Keqiang set a target of around 7.5% growth this year.
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China's GDP growth has been falling since 2010, when it stood at 10.4%.