The yen surged Friday after the Bank of Japan made minor tweaks to its stimulus programme, disappointing dealers who had expected a big announcement to kickstart growth in the world's number three economy.

The central bank said it would boost its exposure to riskier investments, but left a massive 80 trillion yen ($772 billion) annual asset-buying programme unchanged. The dollar sank to 103.25 yen from 104.20 yen earlier.