It won't quite be hand-to-hand combat, but 'currency wars' will come to Moscow on Friday as finance officials from the Group of 20 nations spar over Japan's expansive policies that have driven down the value of the yen.

The G-20 forum, which put together a huge financial backstop to halt a market meltdown in 2009, is back in the spotlight after a week in which the Group of Seven rich nations tried, and spectacularly failed, to speak on currencies with one voice.