ROME The UN world food body reached a landmark agreement on Sunday to try to save the world's diversity of agricultural crops, officials said.

The pact followed an anguished debate pitting many poor countries and environmentalists against multinational corporations and wealthier nations.

After a week of touch-and-go talks, delegates said the United States had agreed for the first time in a public forum to mandatory payments by plant breeders and geneticists developing new crop varieties in return for access to public seed banks.

The seed banks lend out crop seeds at no charge, enabling research into new varieties of plants to increase resistance to disease and ameliorate some of the impact of global warming.

In turn, this helps alleviate hunger in poorer nations.

"This international undertaking is a milestone -- it will allow the conservation of genetic resources for future generations," Jose Esquinas-Alcazar, secretary of the Commission on Genetic Resources for Food and Agriculture, part of the UN Food and Agriculture Organization (FAO), told Reuters.

He said an international agreement to conserve plant genetic resources was needed because agricultural biodiversity was being lost at an alarming rate.

Esquinas-Alcazar estimated that over time some 10,000 plant species had been used for human food and agriculture, but now no more than 120 cultivated species provide 90 percent of human food supplied by plants.

Representatives of 161 countries reached the agreement by consensus in the early hours of Sunday at FAO's headquarters in Rome after tough haggling over the details. But a separate, core issue over the patenting of seeds, where rich and poor nations differ most, failed to be resolved.

The biggest stumbling block was always the patents issue and after much agonized discussion, the meeting decided not to adopt a clause on intellectual property rights that limit access to seeds. the issue will be tackled instead by an FAO conference in November.

Environmental groups say the patenting of food and seeds by multinational companies threatens food security and access by farmers to genetic resources.

The life sciences industry, on the other hand, believes that seed patents are a vital incentive for research.

Sunday's agreement, encompassing 34 nutritional crop groups and 39 forage crop groups, underlined the need to protect farmers' rights, enabling them to save, use, exchange and sell farm-saved seed.

Until now, seed exchanges have operated informally on the principle of "common heritage" -- an agreement that they are a shared international resource.

Change has been forced by the UN convention for biological diversity, which made nations responsible for their own genetic resources.

FAO's November conference is expected to adopt Sunday's agreement, which will then be submitted to national governments for ratification, delegates said.