Arrivals of unwrought copper, including anode, refined and semi-finished copper products were down from 405,000 tonnes in April and down 23.2% from 470,000 tonnes in May 2018.

For the first five months of the year, imports came to 1.95 million tonnes, down 9.4%, the General Administration of Customs data showed.

"[Unwrought copper] imports volume was lower because of a relatively lower domestic copper financing rate," CRU copper analyst David He said.

The fall comes as China imposes tighter restrictions on imports of scrap copper from July 1, with industry sources expecting a sharp slowdown in arrivals of scrap cargoes ahead of that date.

Copper is widely used in construction and manufacturing, and demand for the metal is considered a bellwether for the health of an economy. China's factory activity in May slumped into a deeper contraction than expected, in a potentially ominous sign for demand, at least for unwrought copper.

However, imports of copper concentrate, or partially processed copper ore, were 1.84 million tonnes last month, customs said. That was up 10.8% from 1.66 million tonnes in April and up 17.2% from 1.57 million tonnes in May 2018.

Concentrate imports are up 16.6% so far this year as smelting capacity in China expands.

Meanwhile, China's aluminium exports rose 7.6% month-on-month, the customs data showed.

China, the world's top producer and consumer of the metal used in everything from cars to cans, exported 536,000 tonnes of unwrought aluminium, including primary metal, alloy and semi-finished products, in May.

That compared with 498,000 tonnes in April and was also up 11.7% from 480,000 tonnes in May 2018.

(Reporting by Tom Daly, Dominique Patton and Shivani Singh; editing by Richard Pullin)