As another difficult round of climate talks approached, the United Nations urged rich countries Tuesday to live up to their promises of help for poor nations in the fight against global warming.

Outgoing U.N. climate chief Yvo de Boer said that, to make headway toward a global climate deal, industrialized nations need to come up with the $30 billion in aid they have promised for the next three years.

Also, negotiators need to focus on a “concrete and realistic goal” for the next major U.N. climate conference in Cancun, Mexico, in December, he said.

“Cancun can deliver if promises of help are kept and if promises to compromise are honored in the negotiations,” de Boer said.

Negotiators from around the world plan to meet May 31 in Bonn for two weeks of expert-level talks on the sketchy draft of a new international climate treaty that would take effect after 2012.

De Boer said the session could “significantly advance that text,” but stressed that “higher political guidance is required to find ways forward.”

The talks come about six months after the U.N. climate conference in Copenhagen. Many environmentalists and political leaders had hoped that summit would produce a breakthrough on fighting global warming, but it came up only with a nonbinding political declaration, the so-called Copenhagen Accord.

That included the $30 billion pledge by industrialized countries for the years 2010-2012 to help poor nations fight climate change and cope with its effects, including droughts or floods.

De Boer said that promise needs to be met to build trust.