The Reagan administration charged again today that the Soviet Union accelerated nuclear weapons testing before it began a unilateral moratorium Aug. 6.

Presidential spokesman Larry Speakes made the assertion in responding to remarks Tuesday by Soviet leader Mikhail Gorbachev, who said Moscow had left its testing program "unfinished" when the moratorium began.

"The Soviets have finished the testing and deployment of an entire generation of new missiles," Speakes said, mentioning the SS17, SS18 and SS19.

"They have done substantial testing" on the SS24 and SS25 missiles, he added, "and accelerated their testing" of the two missiles "before the moratorium announcement."

Earlier, a senior administration official said the accelerated testing allowed the Soviets to declare a moratorium for the next five months because they had already finished a planned series of tests for this year. The United States had not completed planned tests, he said, adding that the Soviets also will need to resume testing in the future.

Today, Speakes was asked if the Soviets had finished testing. He said "perhaps they haven't" on the SS24s and SS25s. But he said Moscow had "definitely accelerated" testing "during this time period" just before the moratorium was announced.

The exact U.S. position on nuclear testing was confused recently when President Reagan suggested that the United States might be willing to agree to a permanent moratorium on testing with the Soviets once a series of U.S. tests is complete. The White House later clarified his remark, saying Reagan was not making a proposal and that the United States could not agree to a permanent moratorium until verification problems were resolved.

The Soviet moratorium was announced July 29 and took effect Aug. 6, on the 40th anniversary of the atomic bombing of Hiroshima near the end of World War II.

Dismissing the Soviet action as a propaganda move, the United States has refused to join the Soviet moratorium, saying that both sides need to continue testing. The Soviets announced they had put the moratorium into effect unilaterally until the end of this year and would keep it in effect longer if the United States joined the ban.

In response to Gorbachev's comments to the Soviet news agency Tass, Speakes today reiterated Reagan's recent offer to allow Moscow to witness and monitor a U.S. nuclear test. The Soviets have unofficially rejected the invitation.

Gorbachev said this week that the Soviets "are for verification" of a test ban, but he rejected Reagan's offer of continued testing with on-site inspections. The United States has long said that problems with verification make a test ban undesirable, recalling a Soviet "break out" from the 1958-61 moratorium.

Speakes said Gorbachev's remarks were "mostly a reiteration" of Soviet positions.

"We welcome his recognition of the importance of the verification question in any serious discussion of nuclear testing," Speakes said. But without on-site inspection of tests, he added, the "margin of error in verifying the limits on nuclear testing is quite substantial."