The Carter administration pledged today to seek "a substantial increase" in American foreign aid over the next five years to help create "a new international economic system" more favorable to the world's poorest nations.

But Secretary of State Cyrus Vance, in a speech to a meeting of foreign ministers of the Conference on International Economic Cooperation, placed heavy conditions on his pledge to the independent nations of Latin America, Asia and Africa. The main one was that any new aid would have to be more effectively "planned, delivered and administered."

There was no immediate indication whether the nonindustrial nations attending the conference were satisfied with Vance's pledge, or with the specific proposals and generally more conciliatory tone of speeches of other Western representatives.

The two-year-old conference, also known as the North-South dialogue, is due to end Wednesday if agreement can be reached on a wide-ranging package of economic measures by the "Group of Eight" delegations representing the world's main industrial nations and the "Group of 19" developing and oil-producing countries.

The industrialized nations, led by the American, Common Market, Japanese and Canadian delegations, offered a series of last-minute incentives to the developing countries to set aside demands for radical reforms of the world's trade and financial systems and to declare the conference both ended and a success.

Vance held out the prospect of immediate American aid increases of $375 million as part of a British-proposed $1 billion fund, and expressed American willingness to negotiate a common fund to support price levels for some raw materials.

Canada offered to write off $254 million in loans made to poor countries in Africa and Asia and to take all future aid on a grant basis "if this conference succeeds."

Led politically by such hard-line countries as Algeria and India, but dominated economically by conservative Saudi Arabia, the world's largest oil-exporter, the 19 delegations were caucusing informally tonight to chart their course in the final round of bargaining on Tuesday.

The outstanding differences continue to center on the developing countries' insistence on a comprehensive common fund for commodities and an index to raise prices of raw materials in step with manufactured goods and inflation.

The industrial countries want to split off a separate dialogue on energy supplies and resources, a move that is being strongly resisted by the oil-producers represented in the conference.

The oil-producers do not want to be singled out for such attention. In what appears to be an emerging compromise on this point, Vance and other Western spokesman said today that they favor a continuation of the North-South dialogue in other bodies after the conference ends.

Called in 1975 by French President Valery Giscard d'Estaing to reduce conflict between the world's rich and poorer nations by establishing "a new international economic order," the conference formally began 18 months ago. It started in an atmosphere of confrontation and confusion sparked by global recession, the stunning increases in crude-oil and export prices and the deteriorating economic position non-oil-producing developing countries.

Third World delegates came away from the opening session convinced that then-Secretary of State Henry Kissinger saw the conference as useless, at best, and that the United States would make no concessions in the talks.

Vance's speech did not seek confrontation. It echoed the importance Carter attached in his recent foreign-policy address at South Bend, Ind., to new American efforts to win friends in the Third World.

But Vance also warned the Third World that new standards of efficiency would be set as conditions for future aid.

"President Carter will seek from the Congress a substantial increase in the volume of our bilaterial and multilateral aid programs over the coming five years," he said, "but we will also demand that this aid be more effectively planned, delivered, and administered."