Secretary of State Cyrus R. Vance yesterday announced a complex and highly qualified U.S. pledge not to use nuclear weapons against countries that do not have them.

The pledge, in the name of President Carter, was designed to "extend a feeling of security" to countries that renounce atomic weapons and to improve the U.S. posture in the current Special Session of Disarmament at the United Nations, State Department officials said.

The 73-word pledge will have little practical effect on U.S. policy on the use of nuclear weapons, officials conceded. It does not appear to change the substance of a pledge Carter made at the United Nations last Oct. 4.

Nevertheless, proponents of the pledge within the government expressed hope that the statement will have a positive political impact on Third World nations and particularly those such as India which have not yet signed the Nonproliferation Treaty renouncing the acquisition of nuclear weapons. Vance announced the statement in a brief press appearance just before traveling to Andrews Air Force Base to greet Indian Prime Minister Morarji Desai, who arrived on a state visit.

The U.S. pledge, as announced by Vance, said:

"The United States will not use nuclear weapons against any nonnuclear weapons state party to the Nonproliferation Treaty or any comparable internationally binding commitment not to acquire nuclear explosive devices, except in the case of an attack on the United States, its territories or armed forces, or its allies, by such a state allied to a nuclear weapons state, or associated with a nuclear weapons state in carrying out or sustaining the attack."

At the United Nations last October, Carter said pretty much the same thing, but without specific reference to states that renounce nuclear weapons. His pledge then was:

"We will not use nuclear weapons except in self-defense; that is, in circumstances of an actual nuclear or conventional attack on the United States, our territories or our armed forces, or such an attack on our allies."

State Department officials who briefed reporters following Vance's announcement said the new formulation did not affect the longstanding U.S. refusal to rule out the option of using nuclear weapons against a massive Soviet conventional attack in Europe or a North Korean attack against South Korea.

Asked to cite an example of how the new pledge would make a practical change, an official replied: "If a Ruritanian civilian were to kick a GI, the U.S. would not be barred from using nuclear weapons (under the previous pledge). Under this, we would."

The question of "first use of nuclear weapons has been a major topic at many disarmament meetings. The United States, the Soviet Union and other nuclear powers have been urged to make a clear pledge never to use nuclear weapons against states which do not have them, as part of the political price to be paid to nations which forgo the atomic option.

The Soviet Union has pledged not to use nuclear weapons against a state which has renounced them and which does not have nuclear weapons based on its territory.

The U.S. position has been much more cautious, because of U.S. military alliances and the strategic role accorded U.S. nuclear weapons in deterring an attack by communist powers on U.S. allies.

One of the reasons for yesterday's statement, according to State Department officials, is that some European countries and Japan have been urging some U.S. assurance, as have Third World nations. Such a pledge has also been pushed by the U.S. delegation to the U.N. disarmament session.