Former defense secretary Harold Brown surprised some arms control advocates yesterday by calling efforts to ban all nuclear weapons tests a "red herring" that detracts from attempts to reach more serious agreement on reducing strategic arms.

A comprehensive test ban has "some utility and some risk," he said, "but I don't think it is comparable" in importance to a strategic arms agreement.

Brown's remarks, made during a House Foreign Affairs subcommittee hearing on whether to continue observing the limits contained in the strategic arms limitation talks (SALT) II treaty, were made on the eve of a House vote Thursday to cut off funds for further U.S. nuclear weapons tests as long as the Soviets refrain from testing.

Soviet leader Mikhail Gorbachev has announced that he will end his eight-month test moratorium because of continued U.S. testing.

The third U.S. test of the year is expected to take place at the Nevada Test Site within the next few days, according to informed sources. Code-named "Mogollon," it is a very low-yield test designed to observe the physics of a nuclear explosion, sources said.

Brown, a physicist who once was director of the Lawrence Livermore National Laboratory which designs nuclear weapons, said he favored proceeding with ratification of the 1974 Threshold Test Ban Treaty, which limits tests to 150 kilotons or less. Thereafter, Brown said, he would negotiate reducing the threshold limit to five kilotons. Both superpowers say they are observing the unratified treaty.

On responding to alleged Soviet violations of SALT II, Brown said he favored "moving more quickly on the Midgetman" mobile intercontinental ballistic missile (ICBM), rather than taking any action that would breach treaty limits.

President Reagan is scheduled to meet at the White House today with his top aides to discuss what steps to take in response to what they charge have been Soviet treaty violations. Among the options being considered is drydocking rather than immediately dismantling two Poseidon ballistic missile submarines. That drydocking would put the United States over one of the SALT II limits.

In a related matter, informed sources said the first apparent flight test by the Soviet Union early this month of a follow-on missile to their giant SS18 exploded shortly after launch.