By Richard Black BBC science correspondent

The US team at the Nairobi conference said its farmers needed methyl bromide, but other delegates disagreed.

Developed nations have already cut their use of the chemical by 70%, pledging to phase it out by 2005.

But the row has raised fears that the US may now ignore some of its commitments on ozone protection.

'No alternative'

At the Nairobi conference a number of countries requested small exemptions from the Montreal Protocol, the international treaty on ozone-destroying chemicals.

But the US asked to be allowed to increase methyl bromide use in 2005 rather than eliminating it.

Ozone layer will not be completely healed for at least 50 years

Methyl bromide is used to kill agricultural pests, and US farmers argue there is no effective alternative.

But David Doniger of the environmental group the Natural Resources Defence Council, who was at the talks, said the US government gave in to the demands of business.

"The Bush administration is tilted way over towards the polluters and caters to their wish-list of regulatory weakenings," Mr Doniger said.

"And here you had a section of the chemicals industry and agribusiness saying 'we want ours too'," he added.

Environmentalists concerned

The dispute will now go to a special meeting next year.

US negotiators said they remain committed to the protocol.

But the head of their delegation admitted there would now be pressure inside the US simply to ignore its obligations on methyl bromide.

Environmental groups are concerned that if the US doesn't abide by the Montreal Protocol, some poorer countries will also decide to ignore it.

The ozone layer is starting to heal itself following huge reductions in the use of the most damaging chemicals, CFCs.

But continued use of methyl bromide would significantly lengthen the time it takes to repair itself, and could mean it never returns to normal.