stats bureau@ (Adds details, milestone on Sept. output.)
* Steel output rises 5.3 pct in Sept. From a year ago
* The avg. steel output down 0.5 pct compared to Aug.
* Total Jan.-Sept. output jumps 6.3 pct from a year ago
* Steel mills curb production ahead of winter air pollution curbs
SHANGHAI, Oct 19 (Reuters) - China's steel output in September fell 3.7 percent from the previous month, when output reached a record, as mills in the world's top producer curbed output because of the central government's crackdown on air pollution and overcapacity.
Crude steel production was at 71.83 million tonnes last month, up 5.3 percent from a year ago, and taking the total for the first three quarters to 638.73 million tonnes, up 6.3 percent from a year ago, data from the National Bureau of Statistics showed on Thursday.
The September output was the lowest since February.
The average daily steel output in September was 2.39 million tonnes, down 0.5 percent from 2.41 million tonnes in August, according to Reuters' calculations based on the official data.
Steel rebar futures in China have risen 40 percent this year, driven by government efforts to shut down inefficient and highly polluting steel plants such as induction furnaces since the end of last year.
Chinese mills have ramped up their steel output this year as the higher prices have fattened profit margins. However, output may continue to fall as steelmaking regions shut production lines to comply with government-mandated air pollution targets during the winter heating season.
(Reporting by Ruby Lian and Josephine Mason; Editing by Manolo Serapio Jr. and Christian Schmollinger)