In a statement intended for both Soviet and American ears, Secretary of Defense Harold Brown declared yesterday that the United States will retain the option of deploying a "shell game" mobile missile system in the strategic arms limitation talks agreement being negotiated between the superpowers.

Brown suggested - but did not say explicitly - that the Carter administration would refuse to sign a SALT II pact which forecloses the controversial plan to move around the hundreds of U.S. land-based intercontinental nuclear missiles among thousands of concrete-lined silos.

The purpose of the plan is to make it nearly impossible for the Soviet Union to be certain where the U.S. land-based missiles are at any one time, thus complicating any Soviet attempt to destroy them in a surprise attack.

Such a deployment, which is being advocated by some elements of the civilian and uniformed command of the Carter administration and prominent outside critics of SALT, would also complicate the process of verifying compliance with strategic arms limitations.

Brown's remarks to the national convention of the American Legion, meeting in New Orleans, were described by Defense Department officials as a definitive and authorized restatement of the administration viewpoint on the "shell game" idea, which is also known as the "multiple aimpoint" (MAP) system.

According to Brown, "the parts of the joint draft text of the SALT II agreement that have already been agreed [U.S. and Soviet negotiators] allow deployment of mobile ICBM systems of the type we are considering."

This U.S. interpretation is reported to have been presented by Paul C. Warnke, director of the Arms Control and Disarmament Agency, to Soviet negotiator Vladimir S. Semyonov last month in Geneva. Carter administration officials have been reluctant to characterize "the Soviet response, but one knowledgeable official said the Russians "categorically rejected" the U.S. interpretation.

Brown's statement yesterday, in this perspective, is being read as a shot across the Soviet bow on this issue by high levels of the Carter administration as well as new assurance to domestic backers of the "shell game."

Brown pointed out that "no decision has been made whether or not to deploy mobile ICBM systems" of this or any other sort for land-based missiles, even though the United States maintains that it is permitted this option under the present draft text of the SALT agreement.

He said the "shell game" is only a one of a number of mobile ICBM concepts being evaluated as methods of enhancing the survivability of U.S. land-based missiles as Soviet forces improve in the future.

Any mobile missile-basing system would have to be "fully consistent" with verification provisions of a strategic arms agreement, he said. "The United States will not deploy a mobile ICBM system that would not permit adequate verification of the number of launchers deployed, and other provisions of the agreement," Brown said.

The defense secretary did not explain how a system of the hundreds of missiles rotated among thousands of silos could be adequately verified.

One verification proposal has been to open all the silos for inspection by the other side's satellites at an agreed time so that the missiles could be counted.