U.S. intelligence analysts have tentatively concluded that the Soviet Union has tested a key rocket system covered by the emerging new strategic arms limitation treaty with a warhead capable of carrying more thermonuclear bombs than the treaty would allow.

According to informed sources, the test of a Soviet SS18 missile occurred in December. The SS18 has been tested previously with 10 independently targetable "re-entry vehicles," each of which carries one thermonuclear bomb. The SALT II accord would limit the SS18 to carrying 10 bombs.

The December test showed that the Soviets had altered the SS18 warheads "bus" -- the intricate device that launches each individual bomb -- so that it could now carry more than 10 -- perhaps two to four or more additional bombs, or some kind of decoy material, or both.

The issues raised by this December test are extremely complex, and there may be no ironclad solution to them, according to several sources. The Soviets cannot be accused of violating the SALT limits merely because they have demonstrated the capacity to violate them. But the fact that this capacity has apparently been detected is bound to provoke charges that the Soviets plan to violate the SALT limits.

Paul H. Nitze, a former SALT negotiator and now a leading critic of the emerging treaty, said yesterday, for example, that this latest revelation confirms that important aspects of SALT II cannot be adequately verified by the United States. In other words, the United States now cannot be certain that the Soviets won't deploy more than 10 bombs on the SS18 at some future time.

Under SALT II the Soviets are permitted 300 SS18s, so an additional four bombs on each one would give the Soviets a total of 1,200 more deliverable nuclear weapons than the treaty permits. This would increase Soviet ability to attack American land-based missiles, though to what degree is open to debate among the experts.

The United Satates also tests its warheads in ways that the Soviets could detect as showing the capacity to deliver more than the allowed number of thermonuclear weapons. For example, the warhead on the Minuteman III missile carries three bombs plus various "penetration aids" including decoy weapons and redar-deceiving chaff. One source suggested that any effort to negotiate strict limits on warhead capabilities could interfere with U.S. programs.

The Soviet test was first reported Tuesday in The New York Times in and article that has caused deep concern and some bitter disputes within the national security bureaucracy.Some officials feel the article was inaccurately alarmist, and many officials reportedly believe that it contained extremely sensitive intelligence information that should not have been published.

One authoritative source noted in an interview that the information was so sensitive that it had not previously been mentioned to the Soviets, apparently for fear that informing them would require divulging sensitive information about the way the United States gathers intelligence on Soviet rocket tests.

Several sources refused to discuss this entire issue on the grounds that it was too sensitive. "It really is extraordinary, this kind of stuff leaking out," observed one senior official who is usually willing to talk with some candor about SALT matters.