The Soviet Union "will develop and perfect its strategic offensive arms" rather than negotiate reductions if the United States continues its "Star Wars" missile-defense research program, Col. Gen. Nikolai F. Chervov, a senior member of the Soviet general staff, said here yesterday.

Chervov, accompanying Politburo member Vladimir V. Shcherbitsky in a visit to the United States, said, "We are not going to sit on our hands and wait until you decide whether or not it would be worthy to deploy such a system . . . . You are working on your system, and that means we'll start perfecting our strategic offensive arms.

"This is not something for the future. It is something which is going to happen in practice," added Chervov, whose rank is equivalent to that of a U.S. lieutenant general and who heads the branch of the Soviet general staff with jurisdiction over military participation in arms control negotiations.

Amid final U.S. preparations for the arms talks, President Reagan won a measure of support yesterday for his Strategic Defense Initiative, dubbed Star Wars, from Italian Prime Minister Bettino Craxi in a meeting and working lunch.

Reagan told reporters after the two hours of discussions that Craxi "assured me of Italy's full understanding of the program's objectives, and we agreed on the great potential benefits this program could provide."

Last week, Soviet Foreign Minister Andrei A. Gromyko appealed to Craxi in Rome to oppose the SDI, saying withdrawal of the Star Wars plan is "absolutely essential" for success in the arms control negotiations.

Craxi told a news conference after his White House meeting, "I think the United States will not give up their idea of their research program" on space defense and said "it does not seem correct" to make such a U.S. concession a condition for broader arms control progress.

Craxi added, though, that certain "guarantees" can be realistically considered. Foreign Minister Giulio Andreotti said Craxi was referring to U.S. guarantees that Star Wars research would be done in compliance with the 1972 Antiballistic Missile Treaty and that development and production of Star Wars systems must await agreement on the matter with the Soviets.

Reagan administration objectives in the Geneva talks include major negotiated cuts in offensive nuclear missiles and Soviet acceptance of Star Wars as a research program and ultimately a contribution to global security.

Chervov's statement, in a meeting of members of the Shcherbitsky delegation with Washington Post editors and reporters, was among the most explicit Soviet declarations that continued U.S. pursuit of Star Wars would generate increases in Soviet offensive arms rather than reductions, as the Reagan administration hopes.

Chervov and other delegation members met senior State and Defense department officials Monday to discuss U.S.-Soviet arms negotiations scheduled to begin next Tuesday in Geneva.

Chervov, considered by U.S. specialists to be an authoritative Soviet military spokesman, did not specify in the interview at what point the Soviet missile-modernization program might be accelerated to meet the prospect of a new defensive system.

The Soviets have two new intercontinental ballistic missiles in early production, with Pentagon experts projecting deployment of one by the end of this year. Moscow also has resumed production of an older-model strategic bomber, a long-range cruise missile and two missile-launching submarines.

Chervov said massive research advanced by the administration is an effort to make Star Wars irreversible, so "no matter who is going to be in power, he will have to go ahead with that program after the research of such volume." Chervov said the Manhattan Project, which produced the first U.S. atomic bombs, cost $15 billion in 1986 dollars compared with the $26 billion being asked for Star Wars research.

Chervov contrasted "laboratory" and "drawing-board" research, which he said is difficult to verify, with "experimental and design work, which also involves field testing and does not pose real difficulties in terms of verification."

Speaking of the latter category, he said, "As a representative of our general staff, I can tell you that we'll certainly find a way to respond to that kind of a situation and that response will not at all be to enhance the security of the United States." Rather, he said, this would be part of the "action and reaction which is well familiar to you."

In a related development, the Senate confirmed by voice vote the U.S. negotiators for the Geneva talks, Washington attorney Max M. Kampelman, former senator John G. Tower (R-Tex.) and career diplomat Maynard W. Glitman.