South Korea's foreign reserves increased for the third straight month in September on the back of the increased value of non-US dollar assets and investment gains, central bank data showed Monday.

The country's foreign reserves had come to $419.97 billion as of end-September, up $4.05 billion from the previous month, according to the data from the Bank of Korea.

The central bank attributed the increase to a rise in the converted value of non-dollar assets amid the weak greenback and investment gains.

The dollar index that gauges the greenback's value against major peers fell 0.9 percent last month, boosting the converted value of non-dollar assets, the central bank said.

Foreign reserves consist of securities and deposits denominated in overseas currencies, International Monetary Fund reserve positions, special drawing rights and gold bullion.

Foreign securities, such as US Treasuries, had been valued at $373.3 billion as of end-September, up $3.86 billion from a month earlier. They accounted for 88.9 percent of foreign reserves, the data showed.

The value of deposits stood at $22.28 billion at the end of September, up $240 million over the cited period.

South Korea ranked as the world's ninth-largest holder of foreign reserves at the end of August, the BOK said. (Yonhap)