Finance ministers and central bankers face a tough task coordinating action to spur global growth at G20 meetings this week, with major economies running at different speeds and monetary policies diverging. Concern over the ability of the United States to sustain the global economy as most of the world slows will be high on the agenda as the Group of 20 leading economies hold talks in Istanbul on Monday and Tuesday. The meetings come as Greece casts a new shadow over Europe, cheap oil plays havoc with inflation and growth forecasts and a strengthening dollar threatens emerging economies.