A new Moscow missile?

The United States may have reason to be concerned about possible violations of the 1979 strategic arms agreement. But President Reagan is wisely withholding public castigation of the Russians as long as he is unsure that the US has a firm case. Asked about Soviet treaty compliance in his news conference this week, Mr. Reagan responded carefully, voicing his misgivings and suspicion but declaring that ''you can't go to court without a case and without the solid evidence; and it's just too difficult and we don't have that.''

Such a stance makes it possible to continue probing the issue with Moscow without, however, marring the atmosphere at the arms control negotiations in Geneva. Nothing could be more damaging than loose accusations.

This is not to deny grounds for concern. On Feb. 8 and May 5 the Russians tested an ICBM which the US suspects is the second new type of intercontinental missile tested since the signing of the SALT II treaty. Inasmuch as that pact permits flight testing and development of only one new missile - and the Russians had already designated a larger, solid-fuel missile as the permitted one - it is thought that the missile tested Feb. 8 and May 5 may be ''illegal.'' Not so, say the Russians: It is merely a modified version of the SS-13, a smaller ICBM already in the Soviet arsenal; since SALT II permits certain modifications of existing missiles, the treaty has not been violated. But, the Americans ask, were these modifications sufficient to make the SS-13 count as a new missile?

There is yet no clear-cut evidence one way or the other. Unfortunately, the US radar-equipped tracking ship which helps monitor Soviet missile tests apparently was not on station for the May 5 test. So it could not be fully determined what the missile's characteristics were, or if the Russians had heavily encrypted the data from the tests to prevent US verification - a move that would have violated the SALT II provisions. Also, even the US data obtained on the Feb. 8 test may not have been complete.

One good thing may be said about all this. Both the United States and the Soviet Union want it perceived that they are abiding by the signed but unratified SALT II agreement. The Reagan administration has even taken up the violations issue in the joint Standing Consultative Commission in Vienna, the mechanism set up by the two sides to consider questions of compliance with the SALT treaties. Hitherto, the administration had avoided the SCC with respect to SALT II issues so as not to appear to be giving support to the accord.It is encouraging that the US has now drawn back from making accusations in public, which serves no useful purpose, and opted for discussing possible treaty violations as they should be discussed - quietly and privately through high-level diplomatic channels and in the Consultative Commission.