African nations at last week's special session of the U.N. General Assembly made their most effective case ever for increased western aid to meet chronic economic and development problems plaguing the continent, several U.S. officials and African specialists said yesterday.

The experts cautioned, however, that the week-long session, at which members of the General Assembly voiced unanimous but nonbinding support for a $128.1 billion African recovery plan, may at most be a useful bargaining chip in coming budget cutback battles over U.S. foreign aid.

"I think the session signifies a maturity in the political sense for the Africans," said Robert J. Berg, a senior fellow at the Overseas Development Council. "They came with unity, dignity and substance and they played it in a superb manner. They made their case and they sat back to listen to the world react."

The African plan, formulated by the Organization of African Unity, calls for western donor nations to contribute about a quarter of the $128 billion over the next five years. The U.N. resolution, adopted late Sunday after all-night discussions Saturday, is not binding, and donor nations, including the United States, did not pledge specific amounts of aid.

U.S. officials said yesterday they were highly pleased, however, that for the first time the African nations had pledged themselves to raise huge amounts to spur long-term development and acknowledged past errors in administering their economies and applying foreign aid.

"That forthrightness of taking their own future in hand is something that on this scale I don't remember ever happening," said M. Peter McPherson, head of the U.S. Agency for International Development. He cited recent economic changes in many African countries, including a decline in centralized planning and a willingness to rely more on free-market incentives for agricultural production, as evidence that many African states may have lost interest in blaming their problems on outsiders.

McPherson said he believed the new, unified African stance was motivated by "realism. The international recession, followed by drought and famine -- there have been a series of failures. . . . The statism that followed their independence in a vast majority of countries simply hasn't worked. . . . It's a pragmatic thing."

The African initiative follows decades of steep economic decline in many nations where subsistence farmers refused to try to grow more food to feed rapidly expanding populations. The farmers have lacked economic incentives in countries where food prices were kept artificially low to maintain political quiet.

The U.N. resolution praised recent African policy reforms, including exchange rate adjustments, debt-relief arrangements, wage and salary reductions and public employment freezes. "Though these tasks have been difficult and painful," it said, "African countries have recognized the need to bear the burden."

A congressional battle is looming, however, over simply holding the line on the Reagan administration's nearly $1 billion foreign aid request for Africa, the specialists said yesterday. The prospect of increased African aid, they added, is dim.

"The administration's disposition continues to concentrate potential cuts on countries that are not Israel, Egypt or countries where we have military bases," said a House staff member who asked not to be named. "As a result, African aid is not likely to pick up all that much. In fact, it's likely to decline significantly."

The White House has asked Congress to appropriate $974 million for Africa for the coming fiscal year, including $340 million for development assistance, $410.5 million for economic support, and $223 million for the Food for Peace program. An additional $47 million for the Peace Corps would bring the total to $1.021 billion.

That figure would be up from $872 million appropriated for 1986.

The U.N. action, said the House aide, "is kind of a marker point. The question is whether Congress in the end will summon up the political will to make hard choices and put more into Africa."

Now, said Berg of the Overseas Development Council, "The Africans have thought more about the problems than the donors have. The donors started this. They said last year, 'You have to do major reforms.' The Africans have been reforming like mad out of great necessity. The donors are now falling behind in the implicit bargains they struck."