The chief U.S. delegate to the nuclear arms talks here said today that new Soviet proposals involving a 50 percent cutback in strategic offensive weapons leaves many questions unanswered but "I feel hopeful that maybe we can start to seriously negotiate."

Despite a lack of detail, considerable ambiguity and the overall complexity of the proposals -- which are causing varying interpretations both here and in Washington -- Ambassador Max M. Kampelman said there "is a sense of hope that this is something."

He spoke with reporters after the Soviets had completed two days of initial explanation of their ideas for offensive-weapons cutbacks linked to demands that the Reagan administration halt its efforts to develop a space-based antimissile defense.

The current round of talks is viewed as a crucial prelude to the summit meeting here Nov. 19-20 between President Reagan and Soviet leader Mikhail Gorbachev. Kampelman said that while he believed the forthcoming summit was a "stimulus" to move the Soviets to present their first concrete proposal since negotiations were renewed in March, the complexities made him doubtful that more than just "narrowing of the issues" could be accomplished before the two leaders meet.

Chief Soviet negotiator Viktor Karpov said his side hoped to make progress in the talks here so that the summit "could be successful."

Officials in Washington indicated that three central questions about the Soviet offer remain unanswered, Washington Post staff writer Don Oberdorfer reported: one, how the overall cutbacks would apply to the forces of both sides and specifically what weapons systems would be affected; two, what the many other aspects and conditions of the Soviet offer mean, such as restrictions on new types of strategic weapons, and three, which of the Soviet conditions are negotiable.

These are among the questions reportedly discussed since Friday in two Washington meetings of the high-level Special Arms Control Policy Group and one meeting today of Reagan's National Security Council. Kampelman is flying back to Washington to join these meetings Wednesday.

In Geneva today, U.S. officials said they do not consider the Soviet proposals outlined thus far to be Moscow's "final positions." As one veteran of previous negotiations put it, "Traditionally Soviet proposals are made up of three parts: negotiating fat, propaganda and serious elements. It is now our job to work out which is which."

Kampelman and other officials here said that there are problems with the Soviet proposals in all three negotiating areas -- strategic and intermediate-range offensive nuclear weapons and defensive space weapons. In addition, Kampelman warned that the overall Soviet package seemed "to have a number of conditions attached to it." That was his diplomatic way of noting that the Soviet proposals specifically linked any reductions on offensive nuclear missiles and bombers to an agreement restraining Reagan's Strategic Defense Initiative (SDI), the so-called Star Wars research program.

Today's meeting lasted 75 minutes, 30 of which were taken up by Karpov reading the second half of the Soviet proposal. The proposal includes a freeze on new nuclear weapons along with a moratorium on tests of nuclear weapons.

A proposed Soviet ban on "new types" of strategic weapons already has drawn objections from some officials in Washington. They interpreted it as meaning that the Soviets could go ahead with their new SS24 and SS25 intercontinental ballistic missiles and the submarine-based SSN23 -- depicting them as already underway, modernized versions of existing weapons -- while the United States would be barred from proceeding with the new Midgetman missile, D5 submarine-launched missile for the Trident submarine and the Stealth bomber.

The Soviet proposal to reduce strategic weapons is also said to include the U.S. intermediate-range Pershing II and cruise missiles based in Europe, and land- and carrier-based fighter bombers because the Soviets claim they can strike the Russian homeland. What is missing from the strategic arms proposal, U.S. officials said, was any Soviet reduction of 441 Soviet SS20 intermediate-range missiles that the Russians say cannot reach the United States.

However, the SS20s are reported to be dealt with in another aspect of the Russian proposal, but that is also known to be unacceptable to Washington as presented.

U.S. sources said the delegation would begin questioning the Soviets on the details of the proposals Wednesday. "Some of it is ambiguous," one official said, and in other parts they have linked "categories that are truly different." This process of clarifying the Soviet offer, he said, "could take a long time."

In an interview with reporters before today's special session at the Soviet mission, Karpov reinforced Kampelman's concern about SDI restraints being a condition for any reduction of offensive weapons. The Soviet diplomat, carefully choosing his words, said his proposals touched all three areas of negotiations and considers them in their interrelationship. That is Soviet diplomatic code meaning that the United States will not get the offensive reductions without agreeing to defensive restraints.

One portion of the Karpov interview appeared to support the prospect that the proposals as delivered may not be Moscow's final word.

In his 30-minute presentation yesterday to the U.S. delegation, Karpov called for a ban on all research, testing and development of what the Soviets describe as "space strike weapons." Sources said some U.S. officials have interpreted this key provision as barring even laboratory research and thus a step backward from statements made by Gorbachev in his Time magazine interview published early last month.

Gorbachev said he did not want to ban "research in fundamental science . . . concerning space." What he had wanted was a bar "when they start building models or mockups . . . when they hold field tests of space weapons." Gorbachev said that type of activity "is something that can be verified" and thus prevented.

Karpov said today that the Soviet proposal he delivered "speaks of those developments . . . that are coming out of laboratories, into field exercises or something like that,which can be seen by national means of verification."

In other words, Karpov seemed to be saying that while the official proposal called for a total ban, he and Gorbachev were saying that laboratory research could be carried on, but nothing beyond that.