Prime Minister Brian Mulroney today opened the most ambitious trade negotiations ever undertaken, urging officials of 90 nations to compromise to resolve trade differences. In an apparent reference to a clash between the United States and its allies on farm subsidies, Mulroney exhorted the delegates to ``be motivated by a spirit of honorable compromise.'' Mulroney said farmers around the world ``are looking for results in Montreal. They can no longer survive on a diet of promises.'' The round of trade negotiations is designed to liberalize and broaden the General Agreement on Tariffs and Trade, an international trade-policing treaty signed in 1947. GATT now has 96 members, and 90 trade ministers were at today's meeting. The U.S. delegation has come under strong criticism from Japan, Australia and the European Community over its demand to completely eliminate government supports for farmers. The 12-nation European Community is only willing to reduce them, terming the U.S. position unrealistic. Mulroney urged the negotiators to ``be animated by a spirit of realism,'' echoing similar weekend calls to the United States in preparatory consultations for the mid-term review of the four-year Uruguay Round of trade talks. The United States was further isolated by a tentative accord reached between the Europeans and developing nations that would remove or reduce tariffs on the export of tropical products to the industrialized world. An EC spokeswoman said the United States did not yet support the agreement, which would affect $70 billion in internaional trade of such products as jute, bananas and coffee. Michael Duffy, Australia's Minister of Trade Negotiations, told reporters Sunday that blame for U.S.-European impasse over trade barriers should be shared by the two parties. But he and other officials looked to the Americans to make the first move. Duffy warned that if neither side compromises during the three-day talks, ``it will sour the rest of the round,'' which ends in 1990. The dispute over agricultural subsidies has clouded prospects for progress on other urgent trade problems, such as the inclusion of the booming services trade in the 40-year-old GATT pact and the adoption of safeguards for copyrights and patents. ``It's hard to see how there can be progress in other areas without progress in agriculture,'' said John Crosbie, Canada's Minister for International Trade. The United States took a conciliatory stand but insisted it would stick to its main objective. Chief agriculture negotiator Dan Amstutz noted the United States had moved away from setting a 2000 deadline for elimination of subsidies and agreed to emergency government food stockpiles as long as they are bought on the open market. ``We're flexible,'' he said, but ``we'd rather not have any agreement than a bad one.'' The U.S. delegation also appeared to hold out the possibility it would relent on its rejection of a proposal lifting barriers on the export of tropical products, such as bananas and coconuts, by developing nations. U.S. trade negotiator Alan Holmer warned that failure by the GATT round could create ``uncertainties'' in Congress and within the incoming administration of George Bush. He did not elaborate. But Sen. Patrick Leahy of Vermont, the Democratic chairman of the Senate Agriculture Committee, said failure on the farm support issue would result in an ``export-oriented'' farm bill. Congress is scheduled to draft a farm bill next year and Leahy indicated it would include supports for farmers so they can compete effectively against farmers whose governments also subsidize exports. The United States projects that its 1989 farm supports will amount to $13 billion _ down from the $27 million of 1987 but still a heavy burden on the deficit-ridden budget. U.S. officials say Europeans could save about $200 billion every year if they stopped their farm supports. But the EC, under the influence of powerful farm lobbies, says eliminating supports is not a viable option.