West German Chancellor Helmut Kohl warned the Reagan administration yesterday that U.S. and Soviet proposals for radical reductions in their strategic nuclear forces must take into account the superiority of Soviet conventional forces in Europe to avoid a split in the North Atlantic Treaty Organization.

Speaking at a news conference here about the implications of the Iceland summit for Western Europe, Kohl said, "It must be made clear what that [the proposals] means for NATO." The security of Western Europe and the United States is "indivisible," he said, adding, "You cannot decouple Europe's security from that of the United States."

Unless the issue of Soviet conventional force superiority was "put on the table at the same time" as U.S. and Soviet proposals for sharp reductions in strategic nuclear arms, "NATO's strategy of a flexible response would be deprived of its credibility," Kohl said.

His remarks seem to reflect the emerging European nervousness about President Reagan's offer at Reykjavik to reduce by half all U.S. and Soviet strategic nuclear weapons in five years and eliminate offensive ballistic missiles in 10 years.

The prospect of such an agreement -- with the elimination of all U.S. medium-range missiles from Europe also agreed upon in principle at Reykjavik -- effectively would deprive Western Europe of much of the American nuclear shield that has long made up for the imbalance in conventional forces there between NATO and the Warsaw Pact alliance.

The West German leader said that Reagan had agreed that more attention would have to be given to the conventional force issue and that this would be "particularly necessary if drastic reductions of the nuclear potential of either side would come about."

Kohl said his two days of talks with administration officials, which he said took place in an atmosphere of "trust and friendship," had centered on the agreement in principle between Reagan and Soviet leader Mikhail Gorbachev to remove all their medium-range missiles from Europe and place a global 100-warhead limit on the rest.

He said he and Reagan agreed that "considerable progress" was made in Reykjavik on this issue and that an agreement was "now within reach" if the Soviets did not insist on linking it to an overall accord on a reduction of the two superpowers' strategic nuclear weapons and the president's Strategic Defense Initiative (SDI) research program.

Kohl said that the Soviet linkage of the two arms reduction packages was "contrary" to Gorbachev's statements before the Reykjavik summit and that he saw no reason, "neither political nor in substance," for the linkage. He said he and Reagan had agreed that efforts should be made "to persuade the Soviet Union to give up this position."

Regarding the conflicting U.S. and Soviet views of whether the 1972 Antiballistic Missile (ABM) Treaty allows research and testing of any part of SDI, the chancellor said he had told Reagan that "research" should continue in accordance with the "restrictive interpretation" of the treaty.

This is the basis for current U.S. testing of concepts for the SDI missile defense system, although the administration has said it reserves the right to proceed under a broader interpretation of the treaty with development and testing of exotic technologies, such as laser and particle beam weapons, even in space.

Kohl said he told the president he understood that "research would be carried on within the framework of the ABM Treaty and that this restrictive interpretation, which had been promised and which had been assured, would also continue to be valid. That was the point I made."