By Alex Kirby
BBC News Online environment correspondent
A plea by developed countries to be allowed to go on using a gas that destroys ozone is being debated at an international meeting in Canada.
The treaty has succeeded in reducing destruction of the ozone layer
They want the meeting of signatories to the ozone protection treaty, the Montreal Protocol, to allow continued use of methyl bromide.
The gas, a pesticide for fumigating crops, damages the ozone layer.
It was due to be phased out by 2005, and critics say reprieving it could badly damage the protocol.
The Earth's thin layer of ozone protects all living things against harmful ultra-violet radiation from the Sun, which can cause cancer and blindness and damage the immune system.
Industrial gases, including chlorofluorocarbons (CFCs) and halons, had been eating away at the layer until the ozone "hole" was discovered above Antarctica in the 1980s.
But the protocol has succeeded in reducing the destruction, and on present trends the ozone layer should be returning to normal by about the middle of the century.
The Montreal Protocol does allow continued use of ozone-destroying gases for purposes agreed to be "critical".
The US is asking at the meeting, also in Montreal, for "critical use exemptions" which environmental groups say would in fact expand its use of methyl bromide.
Other countries pressing for similar exemptions include Australia, Belgium, Canada, France, Greece, Italy, Japan, the Netherlands, Portugal, Spain and the UK. The use of the gas had been cut to 30% of the high 1991 levels.
Nick Nuttall, spokesman for the United Nations Environment Programme, said: "We need to see a commitment to a declining trajectory for methyl bromide. Otherwise we're left with a job unfinished.
"If this happens it may send the wrong signal, and so other aspirations and goals like delivering safe and sufficient drinking water in the Third World, reversing the loss of the world's wildlife, and fighting global warming can also be put on hold."
David Doniger, of a US lobby group, the Natural Resources Defense Council, said of the American request: "It's the first time any country has proposed to reverse the phase-out and increase the production of a chemical that's supposed to be eliminated."
Some environmentalists believe the US administration is acting under pressure from the powerful farming lobby in key electoral states like California.