Soviet leaders "might . . . increase their offensive forces" as an initial response to President Reagan's "Star Wars" Strategic Defense Initiative (SDI), Undersecretary of Defense Fred C. Ikle told a Senate armed services subcommittee yesterday.

"It is conceivable," Ikle added, in answer to questions by Sen. Jeff Bingaman (D-N.M.), "they may shift to [building more] bombers and cruise missiles," which would not be affected by a Star Wars system, "if they see our [ballistic missile] defenses are working."

But, Ikle maintained, Moscow eventually would realize that, faced with a capable U.S. space-based defensive system, it would be in their interest to reduce offensive missiles.

Ikle and Lt. Gen. James A. Abrahamson, director of the SDI program, who appeared with him, were questioned sharply by panel Democrats about what were described as inconsistencies in Reagan officials' descriptions of the program and how it would be handled at the upcoming arms control negotiations in Geneva.

In answers to Sen. Carl Levin (D-Mich.), Ikle said U.S. negotiators would "discuss how we can walk toward a regime of missile-defense systems together," but he repeated the president's statement that the United States would not agree to limiting Star Wars research.

When Levin pressed him to say what the Soviets could expect from the Geneva arms control negotiations concerned solely with the systems generally grouped under the SDI heading, Ikle responded, "confidence-building measures" and "how we can coordinate phasing in defensive systems."

Chairman John W. Warner (R-Va.) told Ikle that his remarks had created a negative atmosphere about the administration's approach to Geneva. Warner said he believed that the United States is prepared to discuss testing and deployment of space defense systems and asked Ikle to comment.

"It would not be a good idea to speculate" on the American negotiating position, Ikle said, repeating that "short-term Soviet violations" of the ABM treaty would be brought up.

Sen. Edward M. Kennedy (D-Mass.) declared that the Reagan administration was "telling the Soviets to abide by the ABM treaty" and at the same time saying "we intended to break out on our own" when we deploy a space-based Star Wars defense system.

Ikle responded that "we are abiding by the treaty" with the current research program and that in the future, "we are proposing to renegotiate its provisions, not violate it clandestinely" as he said Moscow was doing.

Sen. Gary Hart (D-Colo.) told Ikle there was "a difference in tone" between his statement before the committee and a speech delivered Wednesday by Ambassador Paul Nitze, special advisor to the Secretary of State on arms control. Ikle told the subcommittee that SDI already was "the very core of our long-term policy for reducing the risk of war," while Nitze, according to Hart, emphasized that missile defense would be important "if it were successful."

Ikle modified his statement to say SDI would be "of central importance if it proves possible."

He also refined Reagan's statement during the presidential debates that the United States would be willing in the future to share information on defense technology.

That would come about, Ikle said, "when the Soviets agree to abolish all offensive systems . . . then we would agree to work together."