Rich countries must provide extra cash to expand a fledgling U.N. fund to help poorer nations adapt to global warming, the U.N. climate change chief said on Tuesday.

The U.N. Adaptation Fund, which is financed by a green tax, has just 37 million euros (58 million dollars) in the pot at the moment. Yvo de Boer, head of the U.N. Climate Change Secretariat, told AlertNet that hundreds of millions of dollars would be needed each year to kickstart climate change adaptation, which includes measures such as switching to new crops, building flood defences and preparing for shifting patterns of disease.

"I think it would send a huge signal of encouragement to developing countries if rich nations were also to significantly contribute to the fund to show that they are taking the question of adaptation seriously," de Boer said on the sidelines of a Geneva conference on the humanitarian impacts of global warming.

The fund, launched in December, is financed with a 2 percent levy on transactions under the clean development mechanism (CDM), which gives carbon credits to investors in projects that cut greenhouse gas emissions in developing countries, such as wind farms and hydro-power projects.

Given the number of CDM projects in the pipeline, the fund is expected to increase to an estimated $80-300 million in the period up to 2012. But this amount is paltry compared with a U.N. call for at least $86 billion in new financing by 2016 to help the world's poor deal with the consequences of global warming such as floods and droughts.

De Boer said wealthy countries should expand the fund with contributions in the form of grants - which should be on top of existing development aid. He declined to say how much rich countries should contribute, because efforts were still under way to work out the cost of adapting to climate change.

But he warned that a failure to provide substantial funding for adaptation using U.N. mechanisms could make developing countries question their wider participation in climate change talks, including reducing greenhouse gas emissions.

The talks are meant to produce a climate deal in Copenhagen in December 2009 to widen and toughen the existing Kyoto Protocol.

The U.N climate chief said he was disappointed with the slow pace of progress towards a new climate pact. In a panel discussion at the Geneva event, he blamed "an unbelievable - almost criminal - lack of leadership" and urged heads of state to put more pressure on their negotiators.

Opening the high-level meeting hosted by the Global Humanitarian Forum, its president and former U.N. Secretary-General Kofi Annan urged the international community to take a proactive approach to tackling climate change by focusing on adaptation.

"This means empowering communities everywhere so that they have the knowledge and the tools available to prepare for the worst effects of climate change, not after its effects have already taken grip, but well before," he told delegates including heads of state and major companies.

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