South Korea on Friday stepped up concern over its surging currency on signs the won's rally to a five-year high against the Japanese yen is starting to hamper exports.

The exchange rate between the two large Asian countries is especially sensitive as they compete head to head in export markets on items from smartphones to cars. While the won has climbed against the US dollar to a two-year high, it has risen more against the yen as Tokyo has sought to push down its currency to help kick start the Japanese economy.