SANTA BARBARA, Calif. (AP) _ The Reagan administration on Wednesday softened an earlier assertion that the Soviet Union had ended a round of nuclear testing before it asked the United States to join a moratorium on such tests.

White House spokesman Larry Speakes was queried about the U.S. claim in light of statements made by Soviet leader Mikhail Gorbachev Tuesday in Moscow.

Gorbachev renewed his call to the United States to join the moratorium, which it had announced on July 29, and rejected the U.S. assertion that Moscow had in fact stepped up its tests just before the moratorium was declared.

Gorbachev said the Soviet Union had left a series of tests unfinished, and that its halt, which was to have gone into effect Aug. 6, was verifiable. He also said the subject of a comprehensive test ban was one that should be discussed at the summit with President Reagan in November.

Speakes said the administration found Gorbachev's statements ''mostly a reiteration of previous Soviet positions.''

But, he added, ''We welcome his recognition of the importance of the verification question in any serious discussion of nuclear testing.

''We will of course study his remarks carefully and may have more to say on the subject when we have done so,'' Speakes said.

The spokesman said that unless on-site inspections are allowed, ''the margin of error in verifying the limitations on nuclear testing is quite substantial.''

Speakes said Reagan's own offer, also made July 29, to have the Soviets visit a Nevada test site and measure the yield of tests ''was designed to improve verification.''

''Such a step could lead to the process of developoing effective verification to help assure compliance with nuclear test limitation agreements,'' he said.

Speakes asserted that the Soviets have finished the ''testing and deployment of an entire generation of new missiles,'' pointing to the SS-17, 18 and 19 missiles.

He said they have done substantial testing of the SS-24s and 25s and had accelerated their testing on such missiles in advance of the moratorium announcement.

But asked whether the Soviets had indeed ended such testing, as administration officials had asserted at the time of the Soviet announcement, Speakes said that on the SS-24 and 25s, ''perhaps they haven't'' stopped testing.

''But they definitely accelerated it,'' Speakes said.

Reagan's offer for an on-site Nevada inspection was unprecedented. Monitoring a test near ground zero would allow the Soviets to calibrate their instruments and thus more accurately measure future U.S. nuclear explosions from afar.