Twelve Republican senators, including two to three whose votes could be crucial, issued a joint statement yesterday on the new strategic arms limitaion treaty (SALT) and indicated serious reservations about key elements of the agreement.

The 12 insisted that they remained neutral and open-minded on SALT II, But adminstration officials acknowledged that if theses senators stick to the positions outlined in their joint statement, all of them will end up voting against SALT or for amendments to it that the administration and the Soviet Union would find unacceptable.

The 12 revealed their position at a press conference yesterday that apparently was timed to coincide with Defense Secretary Harold Brown's major SALT address in New York. The senator's joint statement amounts to a response to the pro-SALT arguments made by Brown yesterday and National Security Affairs Adviser Zbigniew Brzezinski the day before.

Generally, these 12 senators suggested that the SALT pact now all but completed includes loopholes, exceptions and ambiguities that favor the Soviet Union.Several of them used the press conference yesterday to urge the administration to deal with their concerns before submitting a treaty to the Senate "like a tablet from a high," in the words of Richard S. Schweiker (R-Pa.).

Among the 12 signers were Henry Bellmon (R-Okla.), Peter V. Domenici (R-N.M.) and John W. Warner (R-Va.), all of whose names appear on admittedly optimistic administration headcounts of possible SALT supporters. Bellmore, a highly respected moderate, is regarded by many in the Senate and the administration as a particularly significant SALT bellwether.

The other signers are conservatives who were previously counted among likely SALT opponents.

In their joint statement, released in the form of an open letter to all senators, these 12 made the following assertions:

Some aspects of the new SALT agreement will be difficult or impossible to verify using existing means. For example, the range of unpiloted cruise missiles, which would be limited by SALT II, does not depend on any visible characteristics of the missile itself. without new verification measures involving Soviet cooperation, the senator said, any unverifiable limits in SALT II should be dropped from the agreement.

If the Soviet bomber known as Backfire is found by the Senate to have "the capacity to make strategic strikes against the United States," then "it must be limited in the treaty."

According to the Carter administraion, the Backfire does have at least the theoretical capacity to strike the United States, depending on where it is based, how high it flies and whether it is refueled in midair. But the administration regards the Backfire threat as marginal at worst, and has agreed to leave the bomber out of SALT II provided the Soviets give assurances that they will not improve its capabilities or produce more than 30 Backfires a year.

Besides providing for an equal number of strategic weapons or launches for both countries SALT II "must actually provide for real equality in such essential strategic measures as throw-weight," or the total weight of bombs that each side's missiles and bombers can carry.

Like the first SALT agreement on offensive weapons, the SALT II agreement would allow the Soviets to maintain a force of 308 "heavy" or extra-large land-based missiles, a type of weapon the United States had never built or sough to build. This allowance would give the Soviet missile force a much large total throw weight than the American missile force.

SALT II should not restrict the range of American land-based cruise missiles that could be used to strike Soviet targets from West European bases unless it also restricts the SS20, a mobile Soviet ballistic missile that can hit European targets from the Soviet Union.

The 12 senators' letter grew out of a series of meetings organized by Sen. Bob Dole (R-Kan.) in recent weeks. Those who signed, besides the senators already named, include: Alan K. Simpson (Wyo.), Roger W. Jepsen (Iowa), Strom Thurmond (S.C.), Gordon H. Humphrey (N.H.), Malcolm Wallop (Wyo.), William L. Armstron (Colo.) and Harrison H. Schmitt (N.M.).