But their endorsement is conditional on another section - market access for industrial products - which is still being negotiated by leading WTO members.
"They haven't got all that they wanted but they are willing to move the process forward," a delegate from an African country in the G90 group of poorest countries said on Saturday in Geneva.
On the key issue of agriculture, the proposal incorporates an offer by a group of five powerful producers - the United States, the EU, Australia, Brazil and India - to make immediate cuts in trade-distorting domestic farm subsidies.
Brazil's Foreign Minister Celso Amorim, who is leading the G20 group of developing countries, which also includes India, said "small specific problems" for some countries still needed to be resolved before all 147 WTO members gave their go-ahead.
"The text was unbalanced," Amorim said. "Now I think it is balanced within the possibilities."
Asked whether Brazil would accept the agriculture proposal, Amorim said, "If others will take (it), I think we will take it as well.
"I cannot speak in the name of these countries but I do feel much more optimistic than a few hours ago. We will find solutions and there will be an agreement," he said.
"We have advanced by dozens of kilometres compared to Cancun," Amorim said, referring to the ministerial meeting in Mexico last September which failed in a similar venture, opening up a damaging rift between rich and poor countries.
The comments followed a painstaking 13-hour overnight session in Geneva attended by negotiators and ministers from a cross-section of trading nations to find acceptable compromise wording for the controversial parts of the agriculture section.
"The text was unbalanced. Now I think it is balanced within
Brazil's Foreign Minister Celso Amorim
The deal is aimed at re-launching the frozen and behind-schedule Doha round of talks on broadening the reduction of trade barriers, which was launched by WTO ministers in the Qatari capital in 2001.
"Now we have a balanced text. It has improved a lot compared to yesterday (Friday) and the first draft by the WTO," the Brazilian foreign minister said.
Confidence appeared to be growing among diplomats as they advanced towards a self-imposed but delayed deadline to conclude the deal by the end of Saturday.
"On some points we have advanced by 20%, on others by 70% or 90%. In general terms the agreement is satisfactory," Amorim said.
Gregor Kreuzhuber, an EU agriculture spokesman, said, "What we need is a balance of unhappiness."
The EU Agriculture Commissioner Franz Fischler added, "I think we have the capacity to say yes to the agreement [on the agriculture component]."
The EU has given assurances and agreement on eliminating all agricultural export subsidies, provided other rich countries do the same.
Japanese diplomats indicated they were ready to accept, but US Trade Representative Robert Zoellick refused to comment as he arrived.
African countries fear they might
have to reduce tariffs drastically
The negotiators are believed to be turning to their next major obstacle - market access for industrial products, which is primarily a concern for African countries who fear they will have to reduce tariffs more than developed nations.
Other issues that still need to be addressed are trade in services like banking and ways of making customs procedures easier.
The document also proposes, for the first time, a cap on subsidies that are aimed primarily at limiting production, amid concerns by some nations that these have been abused in major industrialised countries.
Any deal has to be agreed upon by all 147 WTO members.