The yen surged more than 2%, for its biggest gain in three years, after Japan intervened in the foreign-exchange market hours after officials delivered a "final" warning to investors against selling the currency.

The Finance Ministry in Tokyo didn't immediately respond to requests for comment. However, Japan's Nikkei newspaper, citing a government official, reported that the government bought yen and sold dollars. Several traders and strategists also said the abruptness of the move indicated action.

Economic officials in the U.S. were notified ahead of Japan's intervention, according to someone familiar with the matter. The effort is in line with a Group of Seven agreement to alert counterparts, and to only act when there's risk of excess volatility.

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