Terming it "a matter of national importance," the Pentagon yesterday announced plans to add still another version of the new and highly accurate cruise missile to the nation's arsenal "as soon as possible."

Unlike other versions already nearing production and meant to carry nuclear warheads over long distances to targets in the Soviet Union, the new version is meant to knock out targets at shorter range, using conventional, or non-nuclear, explosives.

The new missile will have a range of about 300 miles, and the idea is to mount it on many different kinds of U.S. warplanes, allowing them to strike known targets without having to fly very close to them and risk being shot down by enemy defense.

This new project, Air Force officials said privately, could ulitimately become extremely large because the plan is to mount these missiles on Air Force and Navy planes, including the new F16 and F18 fighters, the A6 attack plane, the F111 fighter-bomber and even the P3 patrol plane.

The pentagon announcement said that within the next few days the General Dynamics Corp. of San Diego will be asked to submit a proposal for fullscale engineering development of the new missile. General Dynamics is already building the 1,500-mile range Tomahawk cruise missile, which will carry nuclear warheads in ground and sea-launched versions. The new weapon, which looks like a flying torpedo and is powdered by a jet engine, is to be built from portions of the Tomahawk.

Last week, Boeing won a multibillion-dollar Pentagon contract to build a long-range, nuclear-tipped air-launched cruise missile.

In all of these versions, the key is the computer-controlled guidance system which allows the missile to hit fixed targets with great precision by using its electronic brain to compare the missile's position in flight with the location of the target on the ground.

The technique also will allow aircraft carriers to stay farther off shore, since the missiles, in effect give the planes launched from the carriers the ability to strike targets from farther away.

Pentagon officials say first squadrons should be equipped with these missiles in 1983. By the following year they expect another type of guidance system to be available that could also allow strikes at moving targets, such as enemy ships. This would involve using infrared techniques in an effort to home in on the heat given off by ships under way.