South Korea's foreign reserves fell in May as the dollar's ascent cut the conversion value of holdings in other currencies, central bank data showed Monday.

The country's foreign reserves came to $420.98 billion as of end-May, down $5.7 billion from the previous month, according to the data from the Bank of Korea.

The dollar's rise last month boosted the converted value of holdings denominated in other currencies, the Bank of Korea explained.

The dollar index that gauges the greenback's value against major peers rose 2.6 percent last month, 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 $378.96 billion as of end-May, up $4.62 billion from a month earlier. They accounted for 90 percent of foreign reserves, the data showed.

The value of deposits stood at $17.82 billion in May, down $10 billion from a month earlier.

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