Reagan administration officials yesterday welcomed West Germany's support of a Soviet proposal to bar medium-range and short-range missiles in Europe, but cautioned that negotiations could founder over Soviet insistence on the destruction of U.S. nuclear warheads for 72 short-range missiles owned by the West Germans.

The governing coalition's statement in Bonn yesterday, issued under pressure from others in the Western alliance, emphasized that acceptance of the Soviet proposal must be conditioned on excluding West German missiles from the treaty. The Reagan administration has informally supported the Soviet proposal, but also said during the ongoing treaty negotiations in Geneva that the West German missiles should be excluded.

"I think it's a positive development," Kenneth L. Adelman, director of the U.S. Arms Control and Disarmament Agency, said of the West German statement. "It lets the Soviets know that we are willing to move forward with an agreement on terms that are beneficial to Western security," he said.

Adelman said the dispute over the West German missiles, known as Pershing IAs, "will not be a major obstacle if the Soviets want to move ahead." But he cautioned that "if they want to block an agreement, anything can become an obstacle," including the West German missile issue.

The Soviets have argued that the U.S. nuclear warheads for the West German missiles must be barred because the range of the mobile Pershing IA missiles is 450 miles and all missiles in Europe with a range of 300 to 600 miles would be barred under the agreement under negotiation in Geneva. The warheads are said to have an explosive force of 160 to 400 kilotons, which is equivalent to 160,000 to 400,000 tons of TNT.

The United States, and now the West Germans, have objected that the Pershing IAs and their warheads are "third-country systems" traditionally excluded from U.S.-Soviet arms agreements. Including the missiles would set a bad precedent, U.S. officials said yesterday, by suggesting that thousands of artillery and aircraft -- owned by various members of the North Atlantic Treaty Organization and capable of using U.S. nuclear warheads -- might be subject to superpower negotiations in the future.

For the Germans, the Pershing IAs have long been a symbolic issue. Postwar agreements ban the Germans from owning nuclear weapons, so these missiles, with the U.S. warheads designed for them, are as close as the Bonn government can come to achieving nuclear-power status.

The United States has also objected to the Soviet demand to ban the Pershing IAs because the missiles owned by the West Germans are more than 15 years old, and the Defense Department and contractors would like to sell the West Germans a new rocket, the Pershing IB, with much greater accuracy and lethality, which would be barred if the Pershing IA were eliminated.

No U.S. decision has been made to proceed with development of the Pershing IB, but the Pentagon has begun to press for it and has strongly encouraged the West Germans to hold firm on the issue, according to senior West German and U.S. officials.