President Reagan confirmed yesterday that the Soviet Union informally offered during the last round of arms negotiations in Geneva to reduce its intermediate-range nuclear missiles targeted on western Europe.

But Reagan said the offer, which he did not detail, was not "adequate" because it would leave the United States at a "considerable disadvantage." An aide said later that the administration was still studying the Soviet proposal and would respond formally when the Geneva talks resume late in January.

Sen. Gary Hart (D-Colo.), who heard the Soviet proposal described last month in Geneva, said yesterday it was "a serious one." But he also refused to detail the plan, which was discussed at a lunch he attended in Geneva with Paul Nitze, the chief American negotiator, and Yuli A. Kvitsinsky, head of the Soviet delegation at the Intermediate Range Nuclear Force reduction talks.

Hart said he hoped the Reagan administration, which is known to be split over its next move in Geneva, would not "reject out of hand" the Soviet suggestions.

Reagan stressed to reporters that the Soviet offer came only in response to U.S. plans to modernize its own intermediate-range nuclear arsenal in Europe, implying that American rearmament efforts were having their desired effect.

But Sen. Joseph R. Biden Jr. (D-Del.) warned Defense Secretary Caspar W. Weinberger at a Senate Foreign Relations Committee hearing yesterday that parallel negotiations to limit U.S. and Soviet long-range nuclear weapons would be jeopardized if the United States achieves what Biden called first-strike capability by deploying both the MX land-based intercontinental missiles and Trident D5 submarine-launched missiles.

The combination of MX and Trident D5 gives the United States "a first-strike capability that far exceeds what the Soviet first-strike capability is now, not that's our goal," Biden said. "If that's true, I don't see how we can reach an arms control agreement with the Soviet Union if neither of those items are negotiated."

Both the MX and Trident D5 are known as "hard target killers" because their warheads would have enough accuracy and explosive power to destroy Soviet missile silos and underground command posts. "With MX missile and with the D5," Biden said, "we will be in a position if we were to choose, which we are not going to choose, to inflict serious damage, overwhelming damage upon the Soviet Union if we struck first."

Weinberger declined to discuss such first-strike potential, except to reiterate that striking first is "completely opposed to our doctrine. It's totally opposed to our policy. Our policy is deterrence."

The Soviet offer to reduce intermediate-range missiles in Europe was not made at the bargaining table in Geneva but in informal talks between Soviet and American officials like the one Hart attended.

It forces the Reagan administration to decide whether to pursue it or remain committed to the American "zero-option" proposal that the Soviets remove all their intermediate-range missiles from Europe and cancellation of U.S. plans to deploy new American intermediate-range missiles in NATO alliance countries in Europe.

At yesterday's unscheduled meeting with reporters at the White House, Reagan said the NATO deployment plans, to counter increasing Soviet deployment of SS-20 intermediate-range nuclear missiles targeted on western Europe, had prompted the new Soviet move.

"I think the very fact that they made a proposal to reduce in numbers reveals that they, too, are concerned," Reagan said, "and they became concerned when NATO asked and we agreed to provide comparable intermediate-range missiles... to the SS-20s."

The NATO plan calls for the United States to deploy 108 Pershing II and 464 ground-launched cruise missiles in western European countries, beginning in 1983. The Soviets have strongly opposed introduction of the missiles, particularly the Pershing II, which could hit targets in the Soviet Union from bases in West Germany in less than eight minutes.

The Soviets, however, have over 600 missiles which can hit targets in western Europe from bases inside Russia. "The very fact that they have volunteered [to reduce them]," Reagan said yesterday, "isn't adequate. And it would still leave us at a considerable disadvantage."

The Soviets have proposed, according to administration sources, that both sides reduce their intermediate-range nuclear weapons in Europe to 300 by 1990, with a special sub-limit of 160 missiles each. Because Britain and France already have 162 such missiles, the Soviet proposal would not allow the United States to deploy any Pershing II or cruise missiles.

Some U.S. officials want to reject this idea and stick with the "zero option" that would require the Soviets to remove all their intermediate-range missiles. Others inside the administration, however, are reportedly arguing that it is time to explore a compromise that would allow the Soviets to keep some SS-20s, as they have proposed, but also agree to U.S. deployment of the same number of Pershing IIs and/or cruise missiles.

On Capitol Hill yesterday, Biden warned Weinberger that the Reagan administration must begin efforts to convince western European NATO countries to accept the new U.S. missiles on their soil. Belgium, Denmark and "even Germany," where the government is strongly in favor of the plan, are hesitant, Biden said, and unless the administration begins an effective campaign of persuasion "our European friends are not going to comply with their side of the bargain."

Biden suggested the administration begin to do this by retiring some of the 6,000 nuclear artillery shells, bombs and atomic mines now deployed in western Europe, some of which have been there for over 20 years.

Weinberger responded that American efforts to persuade anti-nuclear Europeans to accept new missiles might be useless. "I don't think it would have any effect on them whatsoever."

Also yesterday, the White House said that the president would not withdraw the nominations Richard R. Burt for assistant secretary of state for European affairs and Robert T. Grey Jr. for deputy director of the Arms Control and Disarmament Agency. Their nominations have been held up in the Senate by opposition from conservative senators led by Sen. Jesse Helms (R-N.C.), who oppose arms control compromises with the Soviets.

The nominations were brought up for a vote on the Senate floor last week and then taken off. Yesterday's White House statement said Burt and Grey "remain the president's nominees for important positions within this administration."