West German Chancellor Helmut Schmidt said today that the North Atlantic Alliance should move ahead with plans to modernize its nuclear weapons base in Western Europe with new U.S.-built medium-range missiles.

The chancellor's comments, in a radio interview, were the clearest public indication of support he has given for the NATO plan -- expected to be adopted formally at an alliance meeting in December -- since Soviet President Leonid Brezhnev's major speech in East Berlin last weekend.

In his speech, Brezhnev had offered to reduce Soviet rockets already aimed at Western Europe but only if the West refrained from deploying similar weapons.

Schmidt criticized the Kremlin's attempt to intimidate the West by threatening retaliation if the alliance move to improve its weapons in the face of a Soviet modernization that is already well ahead of Western efforts. f

Schmidt, however, stressed in the interview that arms control talks with Moscow must move ahead simultaneously with the NATO plan.

"We Germans," he said, especially "strive to negotiate . . . and that means for the good of the alliance as well."

Schmidt said today that if negotiations were successful, not all the modernization plans expected to be decided on by NATO in December necessarily would have to be realized, meaning that there would be time between the December decision to deploy these weapons and actually fielding them beginning in 1983 to negotiate at least some constraints.

Schmidt said he was "very optimistic" that Brezhnev "wants to use the intervening time to negotiate just as the West wishes to use the time between the taking of the decision and its execution to negotiate."

The chancellor previously has pointed out that it was not enough merely to limit the number of Soviet medium-range missiles because Moscow has about 500 much older ones at fixed bases. The problem is that these are being modernized or replaced with new SS20 missiles that are mobile and far more accurate. Each carries three individual warheads.

West Germany's foreign and defense ministers this weekend also called for NATO to move ahead with its plan, but there were also signs of a split between the government and some leading figures in Schmidt's ruling Social Democratic Party, such as former chancellor Willy Brandt and party secretary Egon Bahr, both of whom have voiced more sympathy for Brezhnev's offer.