The non-manufacturing Purchasing Managers' Index (PMI) rose to 55.5 in May from 54.8 in April.
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It comes just days after China reported that the manufacturing sector grew at its fastest pace this year in May.
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China's service sector, which includes construction and aviation, accounts for nearly 43% of its overall economy.
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The PMI is a key indicator of the health of the sector and a reading above 50 indicates expansion.
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The upbeat numbers of both the manufacturing and services PMI come amid the wider economic slowdown in the world's second-largest economy.
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Its economy grew by 7.4% in the first three months of the year, from a year earlier, down from 7.7% growth in the previous quarter.
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China's policymakers have taken various steps in recent months to sustain its high growth rate.
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These include 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.Â