* Jan-Aug fixed-asset investment +7.8 pct y/y (poll +8.2 pct)
* Jan-Aug industrial output +6.0 pct y/y (poll 6.6 pct)
* Aug retail sales +10.1 pct y/y (poll 10.5 pct)
* Jan-Aug private sector FAI +6.4 pct
* Chinese economy faces many challenges - stats bureau (adds details)
BEIJING, Sept 14 (Reuters) - China on Thursday posted its slowest growth in fixed-asset investment in nearly 18 years along with weaker-than-expected industrial output and retail sales, suggesting the economy may be starting to lose steam as lending costs rise.
Fixed-asset investment, a key growth driver for the world's second-largest economy, grew 7.8 percent in the first eight months of the year, the weakest pace since December 1999 and cooling from 8.3 percent in January-July.
Analysts had expected growth of 8.2 percent.
Factory output also disappointed, rising 6.0 percent in August from a year earlier, the weakest pace in nine months, statistics bureau data showed.
Analysts polled by Reuters had predicted factory output would grow 6.6 percent in August, up from 6.4 percent in the previous month.
"In particular, infrastructure spending has now begun to cool as the front-loading of fiscal spending this year means that local governments are now having to pair back their outlays," Capital Economics said in a note.
"With tighter monetary conditions still weighing on credit growth, we expect a further slowdown in economic activity in coming quarters."
The statistics bureau said hot weather and "structural adjustments" weighed on industrial output last month, and added the economy remained on a steady, improving trend. On a monthly basis, output rose nearly half a percent.
Retail sales also confounded market expectations, rising 10.1 percent in August from a year earlier, the softest in six months and cooling from 10.4 percent in July. Analysts had expected a slight pick-up in demand.
Again, however, retail sales rose at a decent clip from a month earlier.
China had posted a raft of better-than-expected data in the first half of this year and in recent weeks, defying analysts' expectations for a slowdown.
A government-led construction boom has lifted demand and prices for everything from cement to steel to glass, helping offset an expected drag from property curbs and a regulatory crackdown on riskier types of financing.
Exports have also rebounded after several lean years due to stronger global demand.
This has helped China's industrial firms post their strongest profits in years.
Earlier data for August had shown the country's producer price inflation accelerated to a four-month high in August, while consumer inflation quickened to a seven-month high.
Imports grew more than expected, but export growth eased.
The steel industry churned out record volumes in July a rush to fill a supply gap caused by government-mandated capacity closures and to cash in on high margins.
Growth of private investment slowed to 6.4 percent in January-August from 6.9 percent in the first seven months of the year, suggesting small- and medium-sized private firms still face challenges in accessing investment-finance.
Private investment accounts for about 60 percent of overall investment in China. (Reporting by Kevin Yao and Lusha Zhang; Writing by Sue-Lin Wong; Editing by Kim Coghill)