The World Bank's soft-loan affiliate, the International Development Association, announced yesterday that 33 nations have agreed to finance it's lending program for the next three years with new resources amounting to $12 billion.

Technically known as the "sixth replenishment" for IDA, the additional $12 billion will enable the agency to finance development projects for the poorest of the poor countries.

The U.S. share of the $12 billion comes to $3.24 billion, maintaining the annual dollar volume of U.S. assistance at about the same levels as recent years but accounting for a reduced percentage of the total IDA burden, from 31.2 percent to 27 percent.

Major percentage increases have been negotiated by IDA for Japan (from 10.3 to 14.65 percent) and for West Germany (from 10.9 percent to 12.5 percent).

The U.S. Congress and other legislative bodies must approve the contributions. IDA said that the additional money, for the three-year period beginning July 1, 1980, will allow for an increase in IDA's resources, even after inflation. The fifth replenishment was for $7.5 billion.

If the U.S. Congress fails to put up the full U.S. share as negotiated, the other nations' contributions would be reduced proportionately, a bank aide said.

A potential hurdle to U.S. participation was cleared when bank President Robert S. McNamara assured Congress last month, in an unprecedented letter to Sen. Russell Long (D-La.), that IDA would make no loans to Vietnam.

The monies that IDA lends to developing countries are said to be the largest source of multilateral help on concessional terms. The IDA credits run for 50 years, with repayments beginning after 10 years. The loans are interest-free, except for a service charge of 3/4 of one per cent.

Through mid-1979, IDA had extended credits totaling $16.7 billion for projects in 74 countries in Asia, Africa, the Middle East, and Latin America.

Although pressure has been brought on the oil-cartel countries to increase their participation in IDA, the contributions for the sixth replenishment are modest in relation to their wealth and come from only three OPEC countries: $390 million from Saudi Arabia, $200 million from Kuwait and $79.2 million from the United Arab Emirates. Small contributions also were made for the first time by Argentina, Brazil, Greece, Mexico, Portugal and Venezuela.