African nations must await a global trade deal before Washington will cut farm export subsidies, the US has said.
Mike Johanns says a WTO deal is needed to end the subsidies
At the G8 summit earlier this month, President Bush vowed to eventually lift the farm subsidies.
But failure to secure a global deal at the World Trade Organisation meeting in December could see the subsidies extended for years, trade chiefs said.
They were addressing member countries of the African Growth and Opportunity Act (Agoa) at a forum in Senegal.
The forum, involving workshops and networking events, was designed to encourage African countries to sell efficiently in the US marketplace.
African officials had been pushing for a timetable for the ending of farm subsidies at the meeting in Dakar.
But US Agriculture Secretary Mike Johanns said they could not expect one until other rich countries agreed to end their own subsidies under the so-called Doha round of WTO talks in Hong Kong in December.
"If we do not (complete Doha) a new farm bill will be set in place for a number of years and we will have lost the opportunity quite literally into the next decade," Mr Johanns said.
Agoa, signed in 2000 by Bill Clinton, gives exports of Africa's member countries duty-free status in the US market.
President Bush has extended their duty-free status through to 2015.
Since the act was signed, the value of African exports to the US has risen to $26bn, while African countries import $8bn worth of US goods.
Petroleum products make up the bulk of African exports to the US, but exports of textiles, minerals and agricultural goods have picked up.
However, agricultural subsidies favouring US farmers have dealt severe blows to African efforts to compete in the food market.