(Reuters) -The International Monetary Fund on Thursday warned of "uncertainty" around the direction of Japan's monetary policy, saying a possible shift from ultra-low interest rates could have a significant impact on global financial markets.The director of the IMF's Asia and Pacific Department also highlighted risks surrounding Asia's economic outlook, including weakening exports, slowing productivity in China, and the fragmentation of global trade. There is a risk that the global economy fragments into trading blocs, which could deal a particularly heavy blow to export-reliant Asia.