* China property slowing under pressure from cooling measures
* Slowdown of key sector seen dragging on broader GDP growth
* Sept sales by floor area fall for first time since early 2015- Reuters calculation
* Sept property investment 9.2 pct vs +7.8 pct y/y in Aug - Reuters calculation
* New construction starts +1.4 pct y/y vs +5.3 pct y/y in Aug - Reuters calculation
BEIJING, Oct 19 (Reuters) - Property sales in China dropped for the first time in more than two-and-half years in September and housing starts slowed sharply, reinforcing expectations that robust growth in the world's second-largest economy is starting to cool.
Real estate, which directly affects 40 other business sectors in China, is a crucial driver for the economy but also poses a major risk as Beijing looks to tame soaring home prices without triggering a crash or a sharp drop in construction activity.
Property sales by floor area fell 1.5 percent in September from a year earlier, compared with a 4.3 percent increase in August and a 34 percent jump in September 2016, according to Reuters calculations based on official data released on Thursday.
That marked the first annual decline since March 2015.
"The negative September sale number shows that, unequivocally, the property boom has peaked," said Rosealea Yao, a property analyst at Gavekal Dragonomics.
"We have seen some big rebounds at the end of the first and second quarter, but given how fast the sale numbers are declining, we expect no big rebound this time."
New construction starts by floor area, a volatile but telling indicator of developers' confidence, rose just 1.4 percent in September on-year, slowing from a 5.3 percent increase in August, Reuters calculated.
The softening in property activity appeared to drag on broader growth in the third quarter, as many economists had predicted. China's economy grew 6.8 percent in the third quarter from a year earlier, easing from 6.9 percent in the second quarter.
Property investment did rise 9.2 percent in September, picking up pace from an expansion of 7.8 percent in August, but analysts noted such investment usually lags sales trends by up to six months.
Investment in the first nine months of the year rose 8.1 percent from a year earlier, compared with 7.9 percent in Jan-Aug. The figure mainly focuses on residential real estate but also includes commercial and office space.
While home prices have levelled off or softened in the biggest cities in recent months in response to a flurry of government cooling measures, property bubbles are still a threat in other parts of the country.
A flurry of small cities have had to unveil fresh property curbs in recent weeks after speculators turned their attention to less-restricted cities that have massive overhangs of unsold houses.
Moreover, high prices are forcing many home buyers to take on more debt, potentially weighing on future household consumption and leaving banks more exposed to any property downturn even as Beijing looks to rein in financial system risks.
Household loans, mostly mortgages, rose to 734.9 billion yuan ($110.80 billion) in September from 663.5 billion yuan in August, despite rising mortgage rates, according to Reuters calculations based on the central bank's data out on Saturday.
Short-term loans also soared in the third quarter, suggesting speculators may be trying to circumvent property cooling measures, economists said.
Policymakers have made stabilising the overheated property market a top priority ahead of a critical Communist Party Congress this week, reiterating the need to avoid dramatic price swings which they fear could threaten the financial system and harm social stability. ($1 = 6.6325 Chinese yuan renminbi)
(Reporting by Kevin Yao and Yawen Chen; Additional Reporting by Stella Qiu; Editing by Kim Coghill)