(Corrects milestone in headline and first paragraph to third weekly gain, not first weekly decline in four)

* KOSPI falls, foreigners net sellers

* Korean won weakens against dollar

* South Korea benchmark bond yield falls

SEOUL, June 21 (Reuters) - Round-up of South Korean financial markets:

** South Korean shares fell on Friday, but were set for their third weekly gain, tracking Wall Street rally this week before losses overnight. The won weakened, while the benchmark bond yield fell.

** The benchmark KOSPI fell 27.10 points, or 0.97%, to 2,780.53 by 04:44 GMT.

** The S&P 500 and Nasdaq closed lower overnight, as market bellwether Nvidia retreated from earlier gains.

** Among index heavyweights, chipmaker Samsung Electronics fell 1.47% and peer SK Hynix lost 2.32%, while battery maker LG Energy Solution slid 1.47%.

** South Korea's foreign exchange authorities said they agreed with the National Pension Service to expand a currency swap line to $50 billion from the current $35 billion to defend the tumbling won against the dollar.

** South Korea's exports for the first 20 days of June jumped 8.5% from the same period a year earlier, customs agency data showed.

** Hyundai Motor shed 1.23% and sister automaker Kia Corp lost 1.53%, while search engine Naver and instant messenger Kakao were down 0.66% and up 0.24%, respectively.

** Of the total 931 traded issues, 261 shares advanced, while 625 declined.

** Foreigners were net sellers of shares worth 139.5 billion won ($100.57 million).

** The won was quoted at 1,388.0 per dollar on the onshore settlement platform, 0.24% lower than its previous close at 1,384.6.

** In offshore trading, the won was quoted at 1,387.6 per dollar, up 0.3% on the day, while in non-deliverable forward trading its one-month contract was quoted at 1,384.9.

** The KOSPI has risen 4.72% so far this year, and gained 2.7% in the previous 30 trading sessions.

** The won has lost 7.2% against the dollar so far this year.

** In money and debt markets, September futures on three-year treasury bonds rose 0.08 point to 105.24. ($1 = 1,387.0500 won) (Reporting by Cynthia Kim; Editing by Rashmi Aich)