Poor nations can benefit from boosting trade among themselves at the same time as they fight for greater access to developed countries and broader worldwide trade, UN Secretary-General Kofi Annan said.
Opening a 180-nation UN trade and development summit in Brazil, Annan told poor nations to increase pressure on wealthy states for access to farm markets and slash tariff barriers to combat what he called "discrimination" in global trade.
But he added that poor nations stand to gain as well from broader trade among themselves.
"Trade among poor countries, in so-called South-South agreements, will not interfere with WTO talks," Mr Annan told delegates in Sao Paulo. "If they reduce tariffs among themselves by half they would get $15.5 billion in additional trade."
The World Trade Organisation's Director General Supachai Panitchpakdi told Reuters on the sidelines of the 11th UN Conference on Trade and Development that rich and poor nations were still far from breaking a deadlock on world trade talks.
Slow progress in so-called North-South talks between developed and developing nations has renewed interest among poorer countries to seek out agreements with other poor countries.
Presiding at the opening ceremony, Brazilian President Luiz Inacio Lula da Silva told delegates developing countries should reduce tariffs among themselves "without having to reduce them to developed countries".
He called for a "new economic and trade geography" favoring poorer people and poorer countries.
"It's a new geography to build the confidence of the majority of the planet. It will soon bring better understanding between rich and poor," said the former metal worker, Brazil's first working-class president.
More than 40 per cent of developing country exports are to other developing countries and trade is increasing at a rate of 11 per cent a year, the UN said.
Poor nations' access to lucrative farm markets is an obstacle as a July deadline looms in the Doha development round of WTO talks meant to create an extra $500 billion for the world economy.
Mr Annan said the Doha round would only succeed if poor nations were granted full access to the markets of the industrialised world and farm subsidies were eliminated.
"Such an outcome would strike a blow not only for coherence, but for development and justice too," he said.
"Coherence" is Unctad's new watchword, a reference to preparing poorer nations' legal systems, infrastructure and business regulations to attract investment and trade in a globalised world.
The European Union has offered to eliminate export subsidies, and reduce other tariffs and barriers to farm trade with poor nations.
The United States said on Sunday it recognised that farm trade was the key to reaching a July deal and would push to prevent another collapse of talks like that in Cancun, Mexico, last September.
As police helicopters buzzed overhead, some 1,000 protesters from Brazilian unions and landless movements marched toward the conference centre, which is surrounded by metal fences and thousands of police and soldiers.