Mexico's chief debt negotiator urged a new international initiative to help write down Third World foreign debt.

The Baker Plan strategy of trying to help highly debted developing nations by lending them more money is not working, said Angel Gurria, the Mexican Finance Ministry's general director of public creditor.Saddling indebted countries with more debt, at high real interest rates, is not going to lead the Third World out of its debt crisis, he told a U.S. Export-Import Bank conference.

A better approach, he suggested, is embodied in U.S. congressional and other proposals to create an international unit to buy discounted Third World debt from creditor banks and reschedule the debt payments on easier terms with borrower nations.

Current large debt repayments, Mr. Gurria said, are preventing Mexico and other countries from achieving satisfactory economic growth. Mexico spends about 5 percent to 6 percent of its gross national product on debt servicing, he said.

An international facility to help write down Third World debt would improve chances of economic growth in the developing nations, he said.

The Reagan administration, for one, vigorously opposes such a proposal. We are discouraged, Mr. Gurria said, that the administration's opposition apparently has succeeded in deleting a debt write-down facility from the omnibus trade bill pending in Congress.

Separately, some U.S. business executives at the Export-Import Bank conference deplored what they regard as inadequate financing, by both the U.S. government and commercial banks, of U.S. exports to developing nations.

The Reagan administration cites budgetary constraints in not expanding Ex- Im Bank's direct credit program. Commercial banks say that developing countries' debt problems are inhibiting the banks' export financing.

David Nicoll, managing director of Combustion Engineering Inc., said the Ex-Im Bank provides only a small fraction of the export credits that some foreign export credit agencies provide.

Byron Jackson, FMC Corp.'s international trade director, said his firm is sometimes forced to export from abroad, rather than from the United States, for lack of U.S. credits.

One banker - R. Michael Rice, an Irving Trust Co. executive vice president - suggested that exporters help overcome their financing problems by forming their own export credit insurance syndicate.

Irving Trust, he said, is working to form such a syndicate with both U.S. and European multinational firms. A number of multinationals have expressed interest, he told reporters.

Both Ex-Im Bank and the Foreign Credit Insurance Association in New York have indicated that they would help the syndicate, Mr. Rice said.