The head of the International Monetary Fund Christine Lagarde has warned central banks not to end their stimulus measures too soon. She said stimulus policies were still needed in some regions, especially Europe, United States and Japan.With signs that the global economy is improving, there has been much debate about when supportive policies should be wound down.The US Federal Reserve has signalled it may slow down its huge monthly bond buying programme, a suggestion that has hit stock markets and currencies around the world.But Lagarde said simulative measures had been successful and were still necessary in key economies.She said any withdrawal should be determined by the strength of individual economies.