The International Monetary Fund yesterday announced a new loan pool of $3 billion to support growth-oriented economic programs in deeply depressed countries, largely in sub-Saharan Africa, over the next six years. The interest rate will be only one-half of 1 percent.

For the next three years, World Bank sources said, additional amounts for the same purpose will be provided as part of the next lending program of the International Development Association (IDA), the bank's concessional loan arm.

In a statement welcoming the new aid for low-income countries with protracted balance-of-payments problems, Treasury Secretary James A. Baker III said that it represented a major step in IMF and World Bank cooperation.

But the plan fell short of Baker's original proposal at the October annual meeting of the IMF and World Bank for a jointly administered special fund for these purposes. Baker withdrew his proposal, acknowledging that it had perhaps been "too innovative."

Opposition to the plan rose among the less-developed nations that feared that it might jeopardize development money they already were receiving. They also were concerned about the severity of loan conditions they might have to follow in a jointly administered operation.

The IMF's money in the new plan will come from repayments into a trust fund that had been created between 1976 and 1981 from the sale of a portion of the fund's gold. According to the IMF, these "reflows" from the first loans will amount to roughly $3.1 billion in the 1985-91 period.

The bank's contribution -- to be administered separately -- will be part of the projected IDA-8 program, assuming enough funds are contributed by participating nations. At a recent meeting of IDA deputies in London, a goal of $12 billion was set for the next three years.

This would encompass $9 billion to match the expiring IDA-7 program; $1.5 billion to duplicate the existing special IDA-related loan facility for Africa; and an additional $1.5 billion to match the new IMF money pool. This proposal will be taken up again in Washington in early April in conjunction with the meetings of the Interim Committee and Development Committee of the organizations. The U.S. Congress has not yet appropriated funds for an IDA-8 program.

If all elements of the proposal are worked out, the low-income countries would be receiving about $1 billion in additional aid over the next three years -- half from the IMF pool and half from the special donations to IDA.

The IMF said that 60 countries would be eligible to get loans from the new Special Adjustment Facility (SAF), but that China and India had said they would not use it, leaving all of the resources available to others on the list.

To tap the SAF, a country must present a "framework" for its policies and objectives, to be developed jointly with IMF and World Bank staffs. Countries whose proposals are approved will receive loans in dollars over three years, equal to 47 percent of their own quota (deposits) in the fund -- 20 percent in the first year, the balance equally divided in the following two years.