By Marijn van der Pas

BONN, Germany, March 25 (Reuters) - Rich countries failed on Saturday to agree with the developing world on how much aid they should pay to help eliminate toxic chemicals which build up in the food chain and threaten the environment.

Delegates from 121 countries were unable to make a breakthrough on stopping the use of so-called "persistent organic pollutants" (POPs) after a fourth round of talks this week in Bonn organised by the United Nations Environment Programme (UNEP).

With a deadline for agreement this year, negotiators are being called upon to decide on provisions on controlling measures for an initial 12 pollutants-including chemicals like DDT and dioxin-listed in the mandate for the treaty.

John Buccini, heading the negotiations for the UNEP, said delegates still disagreed on technical and financial assistance from industrialised countries to fund projects which help developing countries replace POPs with less toxic chemicals.

Buccini said that although some progress had been made on the financing issue, problems remained. He called a further round of informal talks on drafting a UN treaty governing the use of POPs.

"I think that we are still on schedule and that we will be able to conclude negotiations by the end of our mandate," Buccini said.

The fifth and last round of POPs talks is due to be held in December in South Africa. The UN wants a final treaty signing ceremony to be held in May 2001 in Stockholm.

During this week’s negotiations, environmental organisations like Greenpeace and the World Wildlife Fund (WWF) accused the United States of trying to soften the draft text for the treaty.

The WWF said in a statement that while most countries agreed on the words "the ultimate elimination" of POPs during the negotiations, the US and their allies tried to include phrases like "where feasible and practicable" in the draft text.

Responding, the head of U.S. delegation at the UNEP talks, Brooks Yeager, told Reuters: "The treaty has to be technologically and economically feasible.

"Stating unrealistic obligations could keep developing countries from participating as full contributors in the negotiations."

During a previous session, delegates agreed that some exemptions would be needed in phasing production of certain POPs. One example is DDT, a pesticide widely used in Africa and the Middle East to control malaria and similar diseases.

On exemptions of other POPs, Buccini said: "That is one issue that will be solved in South Africa."