SEOUL, June 5 (Xinhua) -- South Korea's foreign reserves marked the first fall in three months last month owing to a lower conversion value of non-U.S. dollar assets, central bank data showed Monday.

Foreign currency reserves came to 420.98 billion U.S. dollars at the end of May, down 5.70 billion dollars from a month earlier, according to the Bank of Korea (BOK).

It turned downward in three months as the strong dollar decreased the conversion value of non-dollar assets.

The dollar index, which gauges the dollar value versus six major peers, advanced 2.6 percent last month.

The country's foreign reserves were composed of 378.96 billion dollars of securities, 17.82 billion dollars of deposits, 14.71 billion dollars of special drawing rights, 4.79 billion dollars of gold bullion and 4.69 billion dollars of the IMF position.

South Korea was the world's ninth-largest holder of foreign reserves at the end of April, unchanged from a month earlier.