South Korea's foreign exchange reserves decreased for the first time in seven months in September amid the expiration of a currency swap deal between Seoul and Beijing.

The Bank of Korea said on Thursday that as of the end of September, the country's foreign exchange reserves stood at 384-point-seven billion U.S. dollars, down 170 million dollars from the previous month. It marks the first drop since February.

The central bank explained that the U.S. dollar's strength reduced the value of non-dollar currencies when converted into the greenback last month. 

The BOK said South Korea was the world's ninth-largest holder of foreign exchange reserves as of the end of August, following China, Japan, Switzerland, Saudi Arabia, Taiwan, Russia, Hong Kong and India.

South Korea's 56 billion dollar currency swap agreement with China expired at midnight Tuesday.