The Reagan administration has told Congress that it is not pursuing negotiations with the Soviet Union on a comprehensive nuclear test ban (CTB) because it needs continued testing to solve "important problems" associated with the U.S. nuclear weapons program.

Up to now, the administration position had been that the major bar to negotiations was the inability to verify a complete ban on underground tests.

A comprehensive test ban would prohibit underground nuclear tests just as existing treaties ban such tests in the atmosphere and in outer space. Some proponents argue that a CTB would reduce the likelihood of nuclear first strikes by lowering a nation's confidence in the reliability of its stockpiled nuclear weapons.

"While a CTB continues to be a long-term U.S. objective," the Arms Control and Disarmament Agency (ACDA) said in an answer to written questions from the House Appropriations Committee, "nuclear tests are specifically required for the development, modernization and certification of warheads, the maintenance of stockpile reliability and the evaluation of nuclear weapons effects."

It added that for a test ban to be effective "It must be verifiable and be concluded under conditions which ensure that it would enhance rather than diminish international security and stability."

In further justification of the administration position, the ACDA statement said that "A test ban could not of itself end the threat posed by nuclear weapons, since it does not deal directly with the kinds, numbers or deployment of nuclear forces."

The agency said that the "immediate U.S. goal" is to reach agreements "which significantly reduce nuclear arms and result in greater stability" and, in the meantime, "it continues to be the policy of the United States to conduct the minimum number of nuclear tests necessary to achieve" solutions to nuclear weapon problems.

The ACDA statement marks the first time the administration has publicly said its desire to continue underground nuclear weapons tests was a reason it halted trilateral test ban negotiations with the Soviet Union and Great Britain.

Little more than one year ago, when the White House announced that it would not reopen talks with Moscow over a CTB, a presidential spokesman told reporters that the problems to be overcome dealt "essentially with verification."

"The Soviets," the official said, "simply have refused to accept measures that would assure effective verification. We won't go on with negotiations until we get verification terms we can live with."

The official suggested that the first order of business in the test ban area would be to "negotiate improved verification procedures" for the threshold test ban treaty signed in 1974 by President Nixon. That treaty, observed though unratified, limits the superpowers to underground tests of 150 kilotons or less.

Privately, according to administration sources, the internal guidance to public officials in 1982 stressed the need for better verification, but also mentioned the need for continued testing. The ACDA statement, sources said, appeared to reverse the emphasis and more correctly represented the view of the Reagan administration.

Critics of the administration's arms control policies have said the White House opposition to both the threshold treaty and the comprehensive treaty stemmed from its desire to keep building weapons.

Under the Carter administration, negotiations on a comprehensive treaty reached a point where the Soviets had accepted in principle the placement of unmanned seismic stations on their territory and Washington had accepted a formula for requested rather than obligatory on-site inspections in the case of possible violations. If one country refused to permit an inspection, however, the requesting nation could withdraw from the treaty.

Negotiations continued despite the Soviet invasion of Afghanistan, and recessed only in November, 1980, after the U.S. presidential election, when Reagan beat Carter.

The new administration's opposition to a comprehensive test ban first appeared publicly in October, 1981, when the then-director of ACDA, Eugene V. Rostow, told the U.N. Political Committee that, despite the "high hopes" that had been attached to a test ban treaty, "A test ban cannot of itself end the threat posed by nuclear weapons.

"International conditions," he went on, "have not been propitious and are not now propitious to immediate action."

Later in 1981, the United States initially refused to participate in the U.N. Disarmament Committee's working group on verification problems. It agreed to the establishment of such a body only in 1982.