Because of an editing error, an article yesterday incorrectly described the Soviet SS-18 missiles disputed in U.S.-Soviet arms control negotiations as "aging." In fact, they are new versions of a weapon system initially deployed in the 1970s. (Published 6/4/90)

The United States and the Soviet Union may have taken important steps away from the Cold War with understandings on trade and weapons last week, but their attempts to reach final agreement on a new strategic arms accord remain mired in some nagging disputes left over from more frigid times.

Progress on the accord has been complicated by two aging Soviet weapons -- the Backfire bomber, and the large intercontinental SS-18 missile -- that have been the subjects of arguments reaching back more than a decade to Jimmy Carter's abortive SALT II treaty. Over the years, the weapons have become potent political issues, capable of inflaming partisans in each country as symbols of strength and weakness.

Senior U.S. and Soviet military leaders no longer refer to each other as enemies -- Joint Chiefs of Staff Chairman Gen. Colin L. Powell said recently that the Soviets should be viewed as our "business competitors." But "there is still an insufficient sense that we're in the 'endgame' {or final phase} of negotiations," one U.S. official said. "Various interest groups on each side are still protecting their hobby-horses, and no one is inclined to shift in the absence of a new message from the top that we want an agreement right now."

A U.S. official who participated in some of the discussions said "part of the problem is that our counterparts are not in a strong position, and in this business it takes strength to make decisions. The events of the last five months have left {Soviet President Mikhail} Gorbachev with little room to maneuver."

No longer do Gorbachev and his aides display their trademark flexibility, the official said. "Now they assert that any compromise will cost them something {in political terms} and they ask, 'Do I have to pay that price and do I have to do it now?' "

More than 40 hours' worth of haggling by experts produced only small gains on four of the five key issues obstructing a new treaty that will trim existing arsenals by 10 to 30 percent. This would leave each side with 8,000 to 10,000 nuclear warheads capable of striking the other's territory, all of which are designed to explode with far greater force than the weapon that devastated Hiroshima in World War II.

But the SS-18 and the Backfire remained major sticking points.

"These are the crown jewels in the eyes of U.S. conservatives," a senior congressional aide said of the Backfire and SS-18 yesterday. "They have been screaming bloody murder about these weapons for a long time."

On the Soviet side, the weapons have also assumed a role in political debate that may exceed their military significance, a Soviet official said. He noted that a key aspect of the Backfire dispute concerns a potential limit on those bombers assigned to attack U.S. naval vessels in a tactical or strategic conflict.

Why, Soviet officials say, should they agree to limit such bombers when the United States has refused to consider any constraints on its own naval arms, as the Soviets have demanded? U.S. officials say that when pressed during discussions this week, the Soviets also repeated a longstanding argument that the range of the Backfire is too short for it to be considered a "strategic" weapon.

"Their whole attitude about the Backfire hasn't changed one whit since the 1970s," a U.S. official complained, adding "we both know we've got a problem to be solved here."

By the end of a special negotiating session involving Soviet Foreign Minister Eduard Shevardnadze and Secretary of State James A. Baker III, the Soviets were willing to declare that the bomber could not be refueled in flight to give it a long-range attack capability, U.S. officials said. But they were not willing to accept a limit on Backfires assigned to naval strike units, and the Bush administration considered this "a loophole" that "no one in the West is willing to go along with," an official said.

The administration also pressed unsuccessfully for stringent limitations on SS-18 flight tests needed to maintain the missiles' reliability. But a U.S. official said the Soviets argued, "Why should we compromise any more? We have already paid enough" by agreeing to halve the number of these missiles, from 308 to 154. The United States has no missiles of comparable size or weight, and will not be required to take any similar step.

"Evidently, Baker wasn't prepared to move any further, because he got savaged {by U.S. conservatives} for taking a few steps toward the Soviet position last month," the congressional aide said. "And Gorbachev had reached his bottom line on arms concessions several months ago."