US Treasury Secretary Timothy Geithner has kicked off the G20 meeting in Nanjing, China with a call for more consistency in international currency exchange rates. Mr Geithner said tight control of currency pricing by some countries was hurting global economy. The United States and other developed nations have been critical of China's exchange rate policy. The meeting is being attended by some of the financial world's biggest names. Mr Geithner said that the Group of 20 nations was working closely to put in place a system that will streamline the exchange rates globally. There have been repeated calls for China to let the value of yuan appreciate against the US dollar. It has been accused of keeping the value of the yuan artificially low in order to help its exporters.