Secretary of Defense Caspar W. Weinberger said yesterday that the United States is "no longer bound" to continue observing the unratified SALT II strategic arms limitation treaty following President Reagan's announcement Tuesday, and other senior U.S. officials described Reagan's decision as among the most important of his administration in the arms control field.

Hardening the impression left by White House officials discussing future adherence to the treaty Tuesday, several officials said the White House announcement went much further in sealing the fate of SALT II than did a tentative decision on the same subject in mid-April that was subsequently discussed with U.S. allies.

The White House announced that the United States would take two Poseidon submarines out of service this spring, thus keeping the country under the SALT II limits for intercontinental ballistic missiles, but added that in the fall, the United States would exceed SALT II limits by deploying new air-launched cruise missiles on B52 bombers. It was this latter aspect of the administration's newly enunciated policy that officials pointed to yesterday in emphasizing that the administration had made an important new policy decision to no longer make weapons-deployment decisions on the basis of SALT II.

Tuesday's more ambiguous statements had drawn political criticism from Republicans, including Senate Majority Leader Robert J. Dole (R-Kan.), who said the decision to dismantle two Poseidons "sends the wrong signal to the Kremlin." Dole called for a more explicit renunciation of the unratified treaty.

At the same time, some leading Democrats had praised President Reagan for Tuesday's statement on grounds that Reagan was staying within the numerical limits of SALT II even while declaring rhetorically that he would no longer recognize them as a "standard" for U.S. strategic weapons deployments.

Among the reasons cited by various officials yesterday for the new SALT policy was Republican political pressure on Reagan, a reported failure of top allied leaders at the Tokyo summit to argue forcefully for keeping SALT II alive, and a sense of Reagan's part that it would be better to take criticism for one big decision on the controversial SALT II issue now than to deal with it piecemeal over several months.

Speaking to reporters before delivering the commencement address at the U.S. Military Academy at West Point, N.Y., Weinberger recalled that Reagan last summer agreed to "go the extra mile" for arms control by continuing temporarily to observe the limits of the SALT II treaty. "The president has traveled that last mile and we are no longer bound by that flawed agreement. It's very simple," Weinberger said.

Senior U.S. arms adviser Paul H. Nitze said in an interview that Weinberger's interpretation is "correct" but added that Reagan is committed "to take another look at the matter" if the Soviets should take the initiative in the next several months to satisfy U.S. concern on arms issues.

Nitze said Reagan has made "a significant decision" by announcing now that the United States will no longer base strategic decisions on SALT II standards. Nitze confirmed that Reagan's decision was "a little bit different" than the tentative mid-April thinking that he and arms adviser Edward L. Rowny took to U.S. allies.

The allies probably would have preferred that Reagan postpone a major decision about SALT II, Nitze said. But he added that in his own opinion "it was wise to make the decision all at once" rather than stringing out the choices over time.

Kenneth L. Adelman, director of the Arms Control and Disarmament Agency, summed up Reagan's decision by saying that "in essence, we're not bound by SALT II any more." The crucial issue before the government was "not the hardware" of temporary compliance with SALT II "but the policy," according to Adelman.

Reagan's decision "in essence changes the president's political commitment," Adelman said.

Washington attorney Paul C. Warnke, who was senior negotiator of the SALT II treaty in the Carter administration, called Reagan's decision "a way to achieve the early demise of arms control" and said it meant that in the administration, "the nuts have won" the policy debate.

Warnke said that after Reagan's statement there is no reason for the Soviet Union to continue abiding by numerical limitations on strategic arms "now that the president has told them that SALT is dead." Warnke said that without SALT II limits, the Soviets could add more than 10,000 ballistic missile warheads and thus greatly increase their military clout.

Reagan said Tuesday that he has ordered dismantling of two Poseidon submarines, which will keep the United States under SALT II limits for the time being as new U.S. forces come into service. But he added that he intends to deploy a 131st B52 bomber with air-launched cruise missiles later this year, which would breach the SALT II limits, "without dismantling additional U.S. systems as compensation."

Some U.S. officials said Tuesday that two more Poseidon submarines might be dismantled early next year, putting the United States once more below SALT II limits. Any such action would be taken for military and economic reasons rather than to stay within the treaty's limits, officials said.

Nitze said yesterday, however, that "the current plan is to recore overhaul and refuel the nuclear reactors aboard those two submarines" rather than dismantle them next year. Because of SALT II requirements, "this would not bring you down below the limits," according to Nitze.