Former U.N. ambassador Jeane J. Kirkpatrick said yesterday that the new U.S.-Soviet treaty to eliminate medium- and shorter-range nuclear missiles should be ratified even though it is likely to weaken NATO and leave Europe more vulnerable to Soviet threats.

Kirkpatrick, whose views on foreign policy have a strong following among conservatives, also suggested in testimony before the Senate Armed Services Committee that the treaty be amended to provide for automatic termination if it is violated by the Soviets.

Committee member Phil Gramm (R-Tex.) said he will propose such an amendment, which would go beyond existing treaty language providing that either side can withdraw from it if "extraordinary events . . . have jeopardized its supreme interests."

Kirkpatrick's reservations about the treaty were the strongest during the opening week of month-long hearings on the Intermediate-Range Nuclear Forces (INF) Treaty signed last month by President Reagan and Soviet leader Mikhail Gorbachev.

Her lukewarm endorsement followed a considerably more favorable review from Carter administration defense secretary Harold Brown, who urged approval of the treaty without substantive change although he, too, said it "might have been possible to negotiate a better one."

Kirkpatrick said she thinks that the treaty "on balance leaves Europe somewhat more vulnerable, the Soviet Union somewhat less vulnerable and the alliance somewhat weaker," even though it looks like a "good deal" on a "weapon-for-weapon" basis.

She said it is "widely felt in Europe that the removal of the {U.S.} Pershing and cruise missiles leaves western Europeans more vulnerable" to Soviet threats by encouraging Soviet military planners to "believe that a conventional land battle can be successfully fought in Europe without risk of a nuclear response."

Many Europeans believe only INF weapons "have the capacity to threaten the Soviet Union without committing the U.S. to all-out nuclear war," she said.

But "it does not follow, however, that the Senate should not ratify the treaty," she added, contending that it would be "profoundly disconcerting" to U.S. allies if the Senate rejected the treaty after the United States encouraged Europe to accommodate to it -- "like enticing someone out on a limb and then cutting it off."

"Damage to the alliance has already been done and nonratification would make it worse," she said.

In his testimony, Brown said he would have preferred a treaty that salvaged some INF weapons but said its political advantages are significant and described it as a "modest but useful step" in U.S.-Soviet relations. "I judge the treaty to be in the interest of U.S. security, and I urge its ratification without actions that would open it to reinterpretation or renegotiation from either side," he said.

Meanwhile, the Reagan administration continued yesterday to deny assertions by Sen. Jesse Helms (R-N.C.) that the Soviet Union has secretly stockpiled a force of SS20 medium-range missiles in violation of the INF Treaty. Maj. Gen. William F. Burns, a senior State Department official nominated to become director of the Arms Control and Disarmament Agency, told the Senate Foreign Relations Committee that "personally I do not agree" with Helms' claims, which are partly based on SS20 estimates by the Defense Intelligence Agency.

"The intelligence reports I have seen do not suggest that the Soviets are trying to get away with anything," Burns said. "I'm satisfied personally they are moving toward compliance with the treaty," by making preparations for the elimination of missiles and launchers covered by the treaty.

Staff writer R. Jeffrey Smith contributed to this report.