"Some see financial stabilisation and growth in some countries. But I want to say this loud and clear: These are merely signs," Ban said.
"For a large number of countries there are no 'green shoots' of recovery. There are only fallow fields. The real impact of the crisis could stretch for years."
Ban also said financial bodies like the International Monetary Fund (IMF) and the World Bank badly need reform but that the world remained divided on how to improve them.
"The world institutions created generations ago must be made more accountable, more representative and more effective," he said.
"I regret that financial institutional reform has divided (UN) member states."
Reform of global financial institutions is one of the key issues set to be discussed at the summit, which is being attended by 142 UN member states.
The summit is set to call for the UN General Assembly to have a greater say in efforts towards a solution to the crisis, rather than the G8 and G20 groups of nations.
Although the financial crisis meeting has been billed as a summit, no leaders of major Western powers are attending.