The United States and the Soviet Union yesterday announced another and perhaps final round of negotiations on a new strategic arms limitation treaty (SALT) amid reports that the two sides have reached agreement on nearly all outstanding issues.

The joint announcement of the Dec. 21-22 talks in Geneva between Secretary of State Cyrus R. Vance and Soviet Foreign Minister Andrei Gromyko was made shortly after President Carter informed a news conference of "good progress" toward completion of a SALT II treaty.

Carter declined to say that agreement has been reached, but declared that the U.S. positions are clear and present harmony among executive branch departments. "If the Soviets are adequately forthcoming, we will have an agreement without further dealy. If they are not forthcoming, then we will continue to negotiate," he said.

Only a handful of issues remained to be resolved after the last round of Vance-Gromyko talks in Moscow late in October. U.S. officials reported that major progress was made on these questions in the course of three meetings at the State Department last week between Vance and Soviet Ambassador Anatoliy F. Dobrymin.

Barrie Dunsmore of ABC News, who has closely followed the strategic arms talks, reported these details of U.S. and Soviet compromises on outstanding issues:

The duration of the protocol covering several particularly sensitive SALT issues will be three years from the time of signing. The Soviet reduction in its strategic nuclear force by 150 missiles, as agreed in the negotiations, will take place during this same period.

The United States will be permitted up to 10 independently targetable nuclear warheads on a new inter-continental missile to be deployed during the period of the treaty, which will run through 1985. The Soviets had proposed a lower limit on such warheads.

U.S. warplanes will be permitted to carry 20 to 30 cruise missiles each. The Soviets had recently proposed an average of 25 cruise missiles per plane, and the Americans an average of 35 per plane, sources said.

All cruise missiles will be assumed to have nuclear warheads, and therefore covered by SALT limitations. The United States had proposed that cruise missiles with conventional high-explosive warheads be permitted without counting against the agreed SALT totals.

Dunsmore reported that the issues remaining to be solved next week concern the Soviet Backfire bomber and the recent Soviet practice of encoding certain missile testing information.

Informed sources said the substance of limitations on deployment of the Soviet Backfire bomber has been agreed upon, but there is still disagreement on the method by which the limitations are to be stated and acknowledge. The Backfire will not be counted as a strategic weapon under SALT.

The Soviets have been infomed, according to officials, that there can be no strategic arms treaty unless they agree to stop encoding performance data sent back to earth during tests of their missiles. There are strong indications that the Russian position on this issue is among the remaining uncertainties about next week's Geneva meetings.

If basic agreement between the two sides can be reached in the Geneva sessions, Vance, and Gromyko are expected to discuss arrangements for a ceremonial treaty signing at a summit meeting of Carter and Soviet leader Leonid I, Brezhnev. Carter repeated yesterday that he hopes for a "broad agenda" of discussions with Brezhnev if and when they meet.

If next week's Vance-Gromyko meeting is successful, the summit signing is expected to take place early in the new year in Washington. Ther are reports that the Soviets prefer February rather than January for a Carter-Brezhnev meeting, and that the White House is considering St. Simons Island, Ga., a presidential vacation retreat, for part of the talks with the Soviet leader.

In the news conference, Carter said the administration is seeking to improve its political and trade relationships with both the Soviet Union and the People's Republic of China, wihin the limits of existing laws and national security policies.

In response to a question about trade with what a newsman called the "major new market" of mainland China, Carter said the establishment of normal diplomatic relations with Peking "would open up increased opportunities for trade with those people."

Growing interest has been registered by some segments of American business in trade wtih Peking. This is being closely watched within the Carter administration because of its bearing on the potential domestic political controversy over full U.S. ties with China.