US Secretary of State Hillary Clinton has promised to help raise $100 billion a year by 2020 to assist developing countries climate change mitigation and adaptation.

The pledge, made in a packed press conference at the UN climate summit in Copenhagen, is conditional however.

Clinton stressed that the money would be going to "the poorest and most vulnerable among us," and was contingent upon a wider agreement at the 193-nation conference covering in particular, transparency.

This was a thinly veiled reference to America's insistence that China accepts an international review of how it controls its greenhouse gas emissions. It also accentuated US climate negotiator Todd Stern's comments last week about China not being eligible for climate change funding.

Clinton said the money would come from a mix of public and private financing but, "In the absence of an operational agreement meeting the requirements I outlined, there will not be that financial agreement, at least from the United States," The pledged amount is below what the European Union has said is required poor countries in Africa, South America and Asia.