GENEVA (AP) _ Superpower negotiations on a Soviet-proposed nuclear weapons test ban began today, with the United States and the Soviet Union disagreeing on priorities.

Negotiators from the two sides met at the U.S. mission. Two cars carrying the Soviet delegation entered the compound at mid-afternoon.

Reporters were not allowed inside the gates and did not have access to any delegation members.

The confidential talks are part of a series of U.S.-Soviet contacts which Washington hopes will help prepare a new summit meeting between President Reagan and Soviet leader Mikhail S. Gorbachev, perhaps before the end of the year.

The Soviets, who have frozen their own atomic testing program, have said they want the talks to focus on a total test ban.

The United States, however, wants negotiators to deal mainly with compliance to existing treaties that limit nuclear explosions to 150 kilotons, or 10 times the yield of the atomic bomb dropped on Hiroshima, Japan in 1945.

Washington and Moscow each claim credit for initiating the negotiations.

A White House statement last week said the United States had ''long sought'' a meeting to present its concerns about the anti-cheating provisions of the 1974 and 1976 treaties on nuclear weapons tests and nuclear explosions.

Neither treaty was ratified by the U.S. Senate but both sides have said they have voluntarily observed their limits.

Reagan said in March that ratification could move forward if the Soviet Union agreed to improve control provisions, including the use of U.S.-developed monitoring equipment.

The official Soviet news agency Tass said Thursday that the meeting was convened at Soviet initiative and would be ''an indicator of the real intentions of the two sides and show who is for and who is against the termination of nuclear explosions.''

Today's meeting comes two weeks before the unilateral Soviet moratorium on nuclear tests is due to expire and a day after the United States exploded its 15th nuclear device since the Soviet freeze on testing went into effect Aug. 6.

Tass claimed the U.S. tests serve to develop arms systems for President Reagan's Strategic Defense Initative, a program designed to develop space- based missile defenses.

The Reagan administration has said nuclear tests will have to continue for the time being, whether or not foolproof safeguards exist against cheating.

Administration officials have said the tests are needed to ensure a safe and effective arms stockpile as long as Western defense depends on a nuclear deterrent.

The U.S. delegation at the testing talks is headed by Robert Barker, a deputy assistant director of the U.S. Arms Control and Disarmament Agency and a a newcomer to superpower talks.

The chief Soviet delegate, Andranik M. Petrosyants, who has ministerial rank as chairman of the State Atomic Energy Committee, is a veteran negotiator. He also took part in the tripartite test ban talks which included Britain and ended in 1980.

Another special meeting began on Tuesday, also in Geneva, to discuss the unratified Strategic Arms Limitation Treaty, known as SALT II, which the United States has announced it will stop observing by the end of the year.

Both sets of talks are conducted separately from the regular U.S.-Soviet arms control negotiations which are in summer recess until Sept. 18.

Soviet and U.S. officials have said no substantive information on the talks would be released while they continue.