The Reagan administration is considering accusing the Soviet Union of a series of arms control violations, including testing two new intercontinental missiles instead of the one the unratified SALT II treaty allows.

Those in the administration who favor making such a charge say they believe, among other things, that it might help forestall further arms agreements on any but the most demanding terms; they say they do not think the Soviets should be trusted to keep their word.

Opponents in the behind-the-scenes discussions counter that Soviet treaty violations would be hard to prove, given ambiguities of language and incomplete intelligence information, and that it might be politically and diplomatically counterproductive for President Reagan to get into yet another public dispute with the Soviets at this time.

Much of the U.S. intelligence community contends that the Soviets tested a second new missile on Feb. 8. The SALT II treaty that the United States has not ratified but that both sides say they are respecting allows only one. The U.S. specialists base their conclusion on the so-called throw-weight or lifting power of the Soviet weapon.

But the Soviets, sources said, have informed the administration that they tested only a modification of their existing solid-fueled SS13 missile in February and thus did not violate SALT II.

The argument turns, in part, on whether the small solid-fuel missile in question had a throw-weight within 5 percent of the SS13's. In a section aimed at limiting new missiles, the 1979 strategic arms limitation treaty, known as SALT II, allows only 5 percent variations in throw-weight.

The new Soviet missile tested in February and again last month went "way over" that limit, one administration official said. But the Soviets coded so much of the telemetry on the test flights that these findings are not considered unchallengeable.

Sources said the February test of the three-stage mobile missile from Plesetsk was successful but the one last month failed. The United States calls the missile PL5 and says it believes it can carry at least four warheads on the basis of incomplete data from the two test flights.

The Soviets in October, 1982, tested what they said was their one new ICBM allowed under SALT II--a big solid-fuel mobile missile like the U.S. MX still in development. It conducted a second test of that multi-warhead missile last winter.

Under SALT II, arguments about treaty violations usually have been threshed out behind closed doors in the so-called Standing Consultative Commission, a U.S.-Soviet body established to assure that arms control rules are obeyed on both sides.

White House spokesman Larry Speakes yesterday confirmed that the Reagan administration may go public on the alleged violations rather than use the commission. He said "interagency groups" have been established to compile possible Soviet violations of SALT II, and of nuclear testing and chemical warfare agreements as well. He said the findings may be made public in a few weeks.

One arms control official from the Carter administration, William E. Jackson Jr., said yesterday that it is "no mere coincidence" that the Reagan administration is considering charging the Soviets with testing two new missiles at the time the president's MX commission is expected to recommend doing the same thing: testing the MX and developing a small, mobile missile.

The administration task force studying Soviet missile tests for treaty violations is expected to meet with Reagan this week, officials said yesterday. Other specialists are concentrating on alleged Soviet violations of the 1974 treaty limiting underground testing of nuclear devices to 150 kilotons or less.

Administration officials said the president has been trying to persuade the Soviets to add on-site inspection provisions to that treaty. The Soviets as recently as last week rejected that idea, they said.

Reagan has laid the groundwork for accusing the Soviets of treaty violations if he decides after the current internal review that this would help his causes.

He said last Thursday, for example: "There have been increasingly serious grounds for questioning their compliance with the arms control agreements that have already been signed and that we have both pledged to uphold. I may have more to say on this in the near future."