Prime Minister Brian Mulroney on Monday opened the most ambitious trade negotiations ever undertaken, urging officials of 90 nations to compromise to resolve trade differences.

In a 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 the 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 willing only to reduce them, terming the U.S. position unrealistic.

Mulroney urged negotiators to "be animated by a spirit of realism," echoing similar calls to the United States in preparatory consultations for the midterm 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.

Michael Duffy, Australia's Minister of Trade Negotiations, said blame for the European-U.S. impasse over trade barriers should be shared by the two parties.