UPDATE 1-China's September iron ore imports rise ahead of winter curbs

* China Sept iron ore imports hit highest since May -calculations

* Steel mills churning out products ahead of winter cuts (Updates with September customs data)

BEIJING, Oct 12 (Reuters) - China's iron ore imports rose to their highest level in four months in September, according to customs data issued on Friday, as steel mills ramped up output ahead of winter production restrictions.

Arrivals of steelmaking ingredient iron ore increased to 93.47 million tonnes last month from 89.35 million tonnes in August, but were down from a record 102.83 million tonnes a year ago, according to data from General Administration of Customs.

September is typically a high season for construction activity in China, with increased demand for both steel products and raw materials.

Earlier China Iron and Steel Association (CISA) data showed that average daily crude steel production at its member mills rose to 1.98 million tonnes over Sept. 1-20, up from 1.91 million tonnes in August and 1.94 million tonnes in July.

"Production restrictions last month were not very strict, so steel mills seized the chance to ramp up output before more intense rules kick in," said Wang Yilin, an analyst at Sinosteel Futures.

Blast furnace utilization rates at steel mills across China reached 68.23 percent in mid-September, the highest in two months, and have been hovering around 68 percent since then, according to data from Mysteel consultancy, as mills rushed to boost output before the production restrictions kick in.

The environment ministry has scrapped blanket cuts from its final winter anti-pollution plan, allowing local authorities to decide the rates and timeframes for individual cuts.

Top steelmaking city Tangshan has started production curbs from Oct. 1, while Handan city - also in smog-prone Hebei province - plans to enforce cuts from Nov. 1.

The winter anti-pollution campaign may also expand to broader regions this year, as governments in the Yangtze River Delta, including the No.2 steel producing province of Jiangsu, work on a similar plan to northern areas.

That could crimp operations at steel mills and activity at downstream users, further curbing demand for steelmaking raw materials including iron ore.

"Steel mills will keep only small inventory when winter cuts kick in, meaning their restocking demand will be dynamic," said Wang.

Vessel-tracking and port data compiled by Refinitiv suggests China will import about 83.46 million tonnes of seaborne iron ore in October, a slight pullback from 85.63 million in September.

For the first nine months this year, China bought a total of 803.34 million tonnes of iron ore, customs data showed.

(Reporting by Muyu Xu and Tom Daly; editing by Joseph Radford and Richard Pullin)