Some administration officials are proposing that the United States keep the 20-year-old, eight-inch nuclear artillery shells deployed in NATO countries. They originally were scheduled to be replaced by the new, controversial neutron rounds.

Informed sources said that dealing with the older weapons is "very sensitive" and that "a flock of options are under discussion."

The ultimate handling of the issue, one top NATO diplomat said yesterday, could create the kind of NATO diplomatic controversy already generated by the neutron question.

There are about 1,000 such aging, high-yield, short-range atomic shells in the hands of American custodial units in several NATO countries, including West Germany, Italy, The Netherlands and Belgium, according to Pentagon sources.

The projectiles are to be fired from eight-inch howitzers manned by U.S. and other NATO forces. The artillery pieces can fire conventional and nuclear artillery shells.

The neutron weapons include eight-inch artillery shells and Lance missile warheads. While the warheads are added to the five-year-old Lance force, the shells were designed as replacements, with the old ones scheduled to be brought back to the United States and disassembled, as is done with all retired U.S. nuclear weapons.

Among ideas now being discussed in the administration for handling the old shells, sources said yesterday, are:

Leaving the shells in place in Europe, even after the entire neutron force has been produced, in hopes they can be traded away as a "bargaining chip" in arms control talks with the Soviet Union.

Opposing this are officials who say the administration is not going to go into, as one said yesterday, "fake trades or bargaining chips," using weapons that the Soviets know are not to be used.

Announcing that old shells will be withdrawn from Europe over the next few years as their replacements are stockpiled in the United States in hopes of easing objections to the decision to produce neutron weapons.

Countering this approach are officials who say that, since the neutron is not being deployed overseas, any withdrawal of old shells would represent "a diminution of the U.S. posture in Europe," in the words of one NATO diplomat.

Taking the shells out of Europe with no publicity as the need develops for the enriched uranium they contain. Little known is the fact that all enriched uranium used in new U.S. nuclear weapons comes from the nuclear materials salvaged from older, retired weapons.

A prime justification for building neutron shells is that military men have come to believe that nuclear shells deployed in Europe since the late 1950s had become unusable.

The administration apparently feels no immediate pressure to decide how the matter will be handled, sources said, because it may be a year or more before production of eight-inch neutron shells reaches a point at which older shells stored in the United States have been replaced.

In contrast to Defense Secretary Caspar W. Weinberger's statement Monday that assembly of neutron weapons had begun, Pentagon and Department of Energy sources said yesterday that it would be "some time" before completed shells or Lance warheads would leave the assembly plant at Amarillo, Tex.

A Pentagon official also said yesterday that Weinberger "misspoke himself or had been misinterpreted by newsmen" Monday when he said the neutron production decision had been made weeks ago.

Weinberger had said the administration decided some time ago to produce components of neutron weapons, but it was not until a top-level White House meeting last Thursday that President Reagan decided to order assembly of parts and production of a completed weapon.

In New Orleans yesterday, Secretary of State Alexander M. Haig Jr. confirmed press reports that he had "some reservations about the timing but not the substance" of Reagan's decision. Sources said Haig had wanted to delay the action until next spring.