* August exports up 15.8 pct yr/yr vs f'cast 19.0 pct rise
* Annual export growth slows for 6th straight month in August
* Row with China clouds outlook for Japan's exports
* Trade surplus falls for first time in 15 months (Adds analyst quotes, details)
By Kaori Kaneko
TOKYO, Sept 27 (Reuters) - Japan's annual export growth slowed for a sixth straight month in August, in a sign that a strong yen and moderating overseas demand could hurt the economy's export-led recovery and prompt the central bank to ease policy further.
Tokyo has also warned that a deepening row over disputed islands with China, the top destination for Japanese exports, has already affected bilateral trade and could hurt Japan's overall growth.
August's 15.8 percent annual rise in exports was less than a median forecast for a 19.0 percent gain, Ministry of Finance data showed on Monday, and far off a peak in February when it rose 45.3 percent, suggesting Japan's economic recovery may lose steam soon.
"We still face a risk of a double-dip recession," said Takahide Kiuchi, chief economist at Nomura Securities in Tokyo.
"There is a possibility that both exports and production data will stay weak in July-September and that GDP could slightly fall in October-December."
U.S.-bound exports increased 8.8 percent from a year earlier, much weaker than a 25.9 percent rise the previous month, hit by a slowdown in shipments of cars to the United States.
Exports to Asia, which account for more than half of Japan's total exports, rose 18.6 percent from a year earlier, slower than a 23.8 percent annual rise in the previous month. Shipments to China climbed 18.5 percent in August from a year earlier, less than a 22.7 percent annual increase in July.
The trade balance came to a surplus of 103.2 billion yen ($1.23 billion), down 37.5 percent from a year earlier and marking the first decline in 15 months. That compared with the median estimate for a 238.4 billion yen surplus and a 44.3 percent rise.
Japan's economy grew a revised 0.4 percent in April-June from the previous quarter, more than an initial government estimate, but slowing exports give policymakers little to cheer about the outlook.
The strong yen also remains a concern for Japan after Tokyo intervened in the currency market earlier this month by selling yen for the first time in more than six years as its surge to a 15-year high against the dollar threatened to derail the fragile economic recovery.
The Bank of Japan will be prepared to act in early October if the fragile economic recovery remains under threat, sources have told Reuters. (Editing by Chris Gallagher)