African countries have called on the US to quickly phase out cotton subsidies after the World Trade Organisation (WTO) ruled that they were illegal.
Cotton growers in West Africa say they have been hurt by US policy
The WTO upheld a complaint from Brazil that the practice depressed world prices and hurt other cotton producers.
The US says it is considering its options after the WTO decision - amid US fears the ruling could impact on its subsidies for other products.
It has 15 months to comply but already fears exist it might delay action.
The WTO's Appellate Body, its top court, said the US policy was "inconsistent" with the global agreement on cotton and that it should be brought "into conformity with its obligations".
A spokesman for the US trade representative said the government was considering all options and that negotiation, not litigation, was the best way forward.
"We will study the report carefully and work closely with Congress and our farm community on our next steps," said Richard Mills.
The WTO ruling could affect the US government's support for other products, such as soya beans.
Cotton producers in West Africa say they have been hit particularly hard by US agricultural subsidies.
Several of Africa's largest cotton producers called on the US to implement the necessary reforms as soon as possible, hailing the WTO ruling as a symbolic triumph.
"It confirms that these subsidies are not fair and must be phased out in a very, very short time," Samuel Amehou, Benin's ambassador to the WTO, told Reuters.
Tanzania said the judgment was "a victory of principles" while Brazil expressed its satisfaction with the ruling and urged Washington to comply "rapidly".
The BBC's Lesley Curwen in Washington says the US response is reinforcing suspicions that it will try to postpone acting on cotton until the current round of world trade talks is complete - which could take until 2006 or even longer.
The US has said that any change would need to be included in any overall Doha free trade agreement, which is not expected until at least December.
If the US does not comply fully, it could face retaliatory sanctions from Brazil and other affected countries and calls for compensation.
The WTO's ruling comes as trade ministers from 30 states meet in Kenya to continue talks to try to reform global farm trade, a central part of the WTO's free trade negotiations.
Speaking in Kenya, EU Trade Commissioner Peter Mandelson said the world's richest nations should provide firm commitments on guaranteeing favourable trade terms for goods from poor countries.