With the clock racing towards a mid-December deadline, ministers from the United States, the European Union, Japan, Brazil and India were to meet at the Indian embassy in London to try to bridge differences before the venue switches to Geneva on Tuesday and more ministers join.
After four years of negotiations, the gap between rich and poor countries, particularly over agriculture, remains wide. All sides have warned the World Trade Organisation (WTO) that negotiations face collapse unless it can be narrowed fast.
Mandelson told EU foreign ministers in Brussels that the EU's offer of average farm tariff cuts of nearly 40 percent was enough to switch the spotlight from agriculture to other areas of the negotiations such as industrial goods and services.
"Having made this offer, Europe is in a much stronger position to negotiate and unlock the talks," Mandelson said.
"We have made others understand that this can now only be a comprehensive negotiation, in which we have to make progress on all fronts simultaneously and with similar levels of ambition," he said, according to speaking notes released to the press.
Launched in 2001, the WTO's Doha Round aims to lower barriers to business across the world economy. Ministers from its 148 member states must approve a detailed blueprint in Hong Kong next month.
WTO Director-General Pascal Lamy, who will be at all the talks, warned that failure would cost the world economy hundreds of billions of dollars in lost trade opportunities, and that poorer countries would lose the most.
Mandelson insisted he had gone as far as he could on agriculture, the sector in which developing countries feel they have most to gain from liberalisation, and that it was time for others to make concessions.
But leading developing nations such as Brazil and India, which head the influential G20 alliance of poorer nations, say they are not ready to make concessions elsewhere until they are satisfied with the farm package.
Brazil, echoed by other farm goods exporters such as Australia, say what the EU has put on the table would exclude most of the products they are really interested in - such as beef, poultry and sugar - from deep tariff cuts.
At the same time, France reminded Mandelson how little room he has for manoeuvre, again warning him it might exercise its veto right over EU acceptance of a world trade deal.
France has accused the Commission of already going beyond the mandate given by EU member states for the WTO talks, specifically on protection against imports for its farmers.
"No one should doubt the determination of France," French Foreign Minister Philippe Douste-Blazy said in Brussels.
The United States, although it has made concessions on farm subsidies, is being asked for more because developing states, notably India, say it can still dump products on world markets.
The G20 and non-governmental organisations accuse the EU and other developed states of trying to get more foreign market access for their own industrial goods and services than they are prepared to concede at home with tariff cuts in agriculture.
"We think the EU proposal on further liberalisation ... purely serves big business," Alexandra Wandel of Friends of the Earth told a news conference in Brussels.