The Soviet Union has deployed 18 more SS20 medium-range nuclear-tipped missiles in the past three months, continuing a steady buildup despite a unilateral Soviet freeze on deployments announced in April, a senior State Department official said today.

The new deployments bring the total number of SS20s, each of which carries three warheads, to 441, said H. Allen Holmes, the State Department's director for political-military affairs. He made the announcement after chairing a meeting of NATO's Special Consultative Group.

The number of SS20s may be of crucial importance to the Netherlands' decision in November on deploying new U.S.-built cruise missiles as part of a NATO deployment. Britain, Italy and West Germany already have the new medium-range missiles.

The Netherlands has said it will install the weapons only if there are more than 378 SS20s on Nov. 1. When the Dutch government, trying to defuse domestic antimissile sentiment, announced the deployment conditions in June 1984, the number of SS20s stood at 378.

A U.S. official at NATO said that despite the Soviet Union's courtship of the West European antimissile movement, Moscow apparently did not intend to let the Dutch influence its deployment schedule.

The Soviet moratorium applied only to European deployments until November. Holmes declined to comment on whether the new deployments were in Soviet European territory, Soviet Asia or both.

The Soviets' call for a matching U.S. moratorium was rejected by the Reagan administration as freezing in place a Soviet advantage.

U.S. officials said NATO had deployed 134 of 572 Pershing II and cruise missiles to counter the SS20s.