The Soviets are constructing a new class of attack submarine featuring a titanium hull and armed with cruise missiles which pose a new undersea threat to the United States, Pentagon officials said yesterday.

The surprise, they said, was not that the Soviets were constructing a new attack submarine in the shipyard near Leningrad but that they took a leaf out of the U.S. Navy's book and made room for a new generation of cruise missiles.

The United States apparently showed the Soviet designers the way by modifying attack submarines of the Los Angeles class so cruise missiles could be stuffed in the bow. The Navy's Tomahawk cruise missiles for submarines can carry either conventional or nuclear explosives.

"It shows that they are taking our submarine cruise missile threat seriously," said one submarine-warfare specialist in commenting on the Soviets' new cruise-missile attack sub.

Once the new attack sub and its sisters become operational, officials said, the Soviet navy will have to develop tactics for using them effectively against the United States. The experts said this will require roaming farther from home ports for long periods than most of the Soviet submarine fleet usually does.

One challenge for the new Soviet attack subs, the specialists added, will be to find ways to foil or elude existing U.S. submarine defenses, including the underwater microphones strung along the ocean bottom off the East Coast and other strategic places.

These microphones transmit submarine sounds to shore stations, where signals picked up by two microphones miles apart can be triangulated to identify not only the submarine's position but its speed and course.

Beyond that vital information, the distinctive sound each submarine propeller makes--called its signature--is stored in a computer bank. The United States has gone to great lengths to record and file these sounds in hopes that every Soviet submarine prowling along the American coast can be identified by class, name and sometimes by skipper.

The identification problem has been eased, submarine-warfare experts said, by the Soviet practice of sending comparatively few submarines along the U.S. coast at any one time.

But this could change, they said, if a large number of the new cruise missile submarines is built.

The U.S. Tomahawk cruise missile comes in three versions. One is for attacking ships up to 300 miles away; a second is for blowing up targets up to 1,500 miles distant with a nuclear warhead, and the third is a conventional warhead capable of flying 800 miles to hit shore targets.

If the Soviets deploy their version of the Tomahawk on the new cruise missile, the specialists warn that practice runs along the east and west coasts can be expected. The closer a submarine gets to a land target such as a bomber base, the less warning there would be of a cruise missile attack.

"You can expect probings along our coasts once these new submarines are deployed," said one expert. This is likely to set off alarms within the Pentagon and Congress. It would be another example of the action-reaction phenomenon of the arms race, as the Soviets do to us what we plan to do to them.