The tentative arms-control agreement virtually pinned down when Soviet Foreign Minister Andrei Gromyko visited the White House does not classify the Russian Backfire bomber as a strategic weapon despite a new U.S. intelligence report showing it able to reach North America with ease.

A top-secret study puts the Backfire's range at over 10,000 kilometers (about 6,200 miles), nearly double some previous estimates. Yet, the strategic arms limitation agreement relies on a Kremlin pledge - clearly unverifiable - not to use its impressive new bomber as a strategic weapon.

That alone would guarantee significant opposition to Senate approval of the SALT II treaty. But briefings within the national-security bureaucracy on the tentative agreement point to the all-too-familiar pattern of U.S.-Soviet negotiations: steady U.S. retreats with no significant Russian concessions.

Senior U.S. officials claim a major "concession" by the Soviets in agreeing to lower the overall limit of 2,400 strategic launchers (including long-range bombers) fixed at Vladivostok in 1975. But since the limit applies to each side, calling it a concession can be challenged.

More significant, the new agreement abondons President Carter's demand of last spring that Moscow effectively limit the number of its fearsome heavy missiles. Without that limitation and in the absence of U.S. heavy missiles, the Russians gave up nothing by reducing the overall limit on strategic launchers.

Similarly, there is a familiar taste to the way the tentative SALT II agreement handles two weapons systems left in limbo at Vladivostok: the U.S. cruise missile and the Soviet TU-26 bomber - the Backfire. The United States will severely limit cruise-missile ranges for three years. But, according to secret briefings, the Backfire will be limited only by "unilateral" Soviet declarations promising not to use it strategically and promising not to increase production.

Reliance on the Kremlin's word collides with highly classified Air Force briefings, featuring a chart showing the Backfire with longer range than the other bombers listed. That chart in turn reflects a recent study sponsored by the Defense Intelligence Agency and carried out by the Air Force's foreign-technology division at Wright-Patterson Air Base.

It reveals that, thanks to important aerodynamic modifications, the Backfire's "B" model - now in serial production - has substantially increased its range. If refueled once, in midair, the Backfire range is 8 per cent greater than the most advanced B-52s and 17 per cent greater than the shelved B-1. The DIA study's conclusion is unmistakable: The Backfire is an intercontinental weapon.

As part of the SALT II agreement, the Russians agree not to refuel the Backfire. But that assurance crumbles on two points: First, the Backfire can hit the continental U.S. without refueling on a one-way mission; second, the Kremlin's promises are simply not verifiable. The "B" model is fitted for mid-air refueling, and advanced Soviet "civilian" aircraft such as the wide-bodied IL-86 can be easily modified to become a tanker.

Nor does the Soviet "unilateral" promise not to increase production really insure against the threat to the United States of a greatly expanded Backfire fleet. Thanks to meager U.S. air defenses, the bomber is believed by the Pentagon to be a much bigger threat than is envisioned by the Arms Control and Disarmament Agency.

Thus, as informally agreed upon, SALT II describes an uneven poker game between the Russian and the American. Each is limited in his betting by table stakes. But when needed, the Russian can reach into his pocket and up the ante - by calling on the long-range Backfire bomber, not included in the overall limit on strategic launchers.

How can the U.S. negotiators countenance this? Senior officials say various U.S. intelligence agencies disagree on the Backfire's range. Yet, a 1976 study performed for the Central Intelligence Agency, which put the Backfire range at 6,000 kilometers (about 3,700 miles), has been discredited as based on faulty premises. In secret congressional testimony July 28, CIA Director Stansfield Turner himself conceded his agency's study was out of date.

U.S. negotiators are putting aside their own intelligence study and accepting Russian promises because they believe that the nation's need for an overall arms-limitation treaty outweighs inequitable provisions it may contain. That was the philosophy espoused by chief SALT negotiator Paul Warnke and his lieutenants in private life, though certainly not by candidate Jimmy Carter or a good many U.S. senators. This contrast promises a historic Senate debate with profound consequences.