The NATO-Warsaw Pact gap in theater nuclear missiles is likely to widen even if the alliance deploys new weaponry in Europe, a defense official warned yesterday.

He said that each Soviet SS20 medium-range missile carries three nuclear warheads, compared with one on U.S. Pershing II and ground-launch cruise missiles set to go to Europe.

Thus, today's Soviet force of about 250 SS20s threatens North Atlantic Treaty Organization nations with 750 nuclear warheads. The Pentagon official, who could not be identified under the ground rules of the briefing, noted that this confronts the alliance with a gap even after the 572 new medium-range missiles are deployed in five European countries.

On top of that, he added, the Soviets might have extra missiles ready for the 250 launchers now deployed, further widening the gap. The SS20 is being produced at the rate of more than one a week, the Pentagon estimates.

Some NATO officials known as the High Level Group will meet in Brussles on June 30, in August and again in September to address the threat posed by Soviet SS20s and possible NATO responses, which include U.S.-Soviet negotiations to reduce the threater nuclear arsenals of both sides.

The defense official said yesterday that the high-level NATO meetings will be designed to help prepare Secretary of State Alexander M. Haig jr. for his meeting with Soviet Foreign Minister Andrei A. Gromyko at the United Nations in late September. Richard Perl, a Pentagon arms control specialist, is the U.S. representative on the alliance's High Level Group. s