Five key senators have told the Bush administration they will not support a new East-West treaty reducing conventional forces in Europe unless the Soviet Union backs down in a dispute with the United States and other nations over its interpretation.

The statements, which came in a letter to Secretary of State James A. Baker III and in a congressional hearing yesterday, appear to doom any chance of quick Senate approval of the accord. Soviet Foreign Minister Alexander Bessmertnykh and other top Soviet officials said during meetings with U.S. negotiators in Washington last week that they did not intend to back down.

Sen. Joseph R. Biden Jr. (D-Del.), who has been designated to chair any Foreign Relations committee hearings on the accord, said the treaty might be approved with a special, legally binding provision demanding Soviet acceptance of the U.S. interpretation.

But Baker and Defense Secretary Richard B. Cheney indicated during separate congressional appearances yesterday that they were reluctant to seek Senate approval until the dispute is resolved. The accord was signed by heads of state from 22 nations on Nov. 19 but has no legal effect until each country has ratified it.

The new warning about the treaty was issued by Sens. Sam Nunn (D-Ga.), John W. Warner (R-Va.), David L. Boren (D-Okla.) and Frank H. Murkowski (R-Alaska), who would play key roles in any congressional review of the accord as the chairmen and senior minority members of the Armed Services Committee and Select Committee on Intelligence.

"We believe it is essential . . . {that the Soviet Union} reverse its unacceptable" position, the senators wrote Baker.

While expressing reservations about the Soviet reading of several treaty provisions, the senators indicated that the biggest problem concerned a Soviet effort to exempt a handful of military units from reductions because they are assigned to coastal defense, amphibious and strategic rocket forces.

The exemption, which primarily covers motorized rifle divisions and infantry brigades stationed near the Baltics and on the Black Sea, would allow the Soviets to keep about 3,500 extra tanks, artillery and armored personnel carriers, in addition to the 46,700 such arms already allowed by the accord.

While U.S. officials said they do not believe these extra units are militarily significant, they noted that allowing the exemption opens a treaty loophole that the Soviets could exploit. They said the Soviets appear to have deliberately reassigned some of their regular Army units to the supposedly "exempt" categories of forces and could do so again.

Representatives from every other nation that signed the treaty, including Moscow's former East European allies, have said they support the U.S.-backed interpretation that would not allow the forces to be excluded. Baker expressed doubt yesterday that these countries would support proceeding toward ratification "before this thing is cleared up."

A separate dispute over Soviet declarations about the overall size of its forces that are subject to the treaty has been eased by new U.S. intelligence data, other officials said. They said a previous estimate that the Soviets had deliberately omitted up to 40,000 weapons was in error and that only 2,000 to 3,000 weapons now appear to have been omitted.

The discrepancy was ascribed to intelligence community confusion about the number of Soviet forces in the zone covered by the treaty, west of the Ural Mountains, on Nov. 19.