The Reagan administration yesterday rejected new Soviet demands for an agreement on 72 U.S. missile warheads based in West Germany, described by Moscow as the last remaining major obstacle to a U.S.-Soviet Euromissiles pact and summit meeting this fall.

White House and State Department officials expressed confidence that the U.S. position, which they described as definite and final, will not interfere with the U.S.-Soviet pact or the summit meeting of President Reagan and Soviet leader Mikhail Gorbachev, despite increasing emphasis on the issue from Moscow.

A senior State Department official called the new Soviet demand "a ploy" to keep controversy alive between the United States and West Germany.

Another official called it "a phony issue" raised by Moscow and said, "They are going to have to back off" if they want a Euromissiles pact.

"We have said continually that third-country systems are not a part of these negotiations and we continue to maintain that position, and we will not negotiate them," White House spokesman Marlin Fitzwater told reporters in Santa Barbara, Calif.

At issue are the U.S. nuclear warheads for 72 West German Pershing IA missiles with a range of about 460 miles, which is in the range category to be covered by the Euromissiles accord under negotiation between Washington and Moscow.

The U.S. administration consistently refused to negotiate on the West German missiles or their U.S. warheads on grounds this is a longstanding "cooperative arrangement" with an ally outside the scope of the U.S.-Soviet treaty.

To give in on this point, U.S. officials said, might set a precedent for inclusion of other allied nuclear weapons, such as those of Britain and France, in U.S.-Soviet negotiations. Washington has adamantly refused to negotiate with the Soviets about such weapons, despite longstanding demands from Moscow.

Last week West German Chancellor Helmut Kohl, succumbing to pressure from Moscow and from his coalition partners in Germany, offered to destroy the Pershing IA missiles once the U.S.-Soviet treaty is signed and implemented. Reagan welcomed Kohl's offer, saying that it removed an "artificial obstacle" to the treaty. The administration did not say what it will do with the U.S. warheads once the missiles that carry them are destroyed.

The United States is prepared to withdraw the warheads from Germany once the missiles have been destroyed and was prepared to say so in recent days, according to State Department sources.

But it will not discuss the matter in the Geneva negotiations and, according to one official, will not engage in "any form of negotiation" about it with Foreign Minister Eduard Shevardnadze when he comes here to meet Secretary of State George P. Shultz Sept. 15-17.

"We don't believe it stands in the way of getting a treaty," Fitzwter said of the Soviet demand for agreement on the U.S. warheads. He said the Pershings are "obviously a concern to the Soviets" but that the U.S. position for "the last several months" that a treaty can be obtained despite this.

Soviet Deputy Foreign Minister Vladimir Petrovsky, at a Washington news conference yesterday, called the forthcoming meeting between Shultz and Shevardnadze "very important" and reiterated the Soviet demand for an accord on the Pershing IA warheads.

At the same news conference, Col. Gen. Nikolai Chervov of the Soviet Defense Ministry repeated the offer he made here Monday: to allow the United States to conduct a nuclear weapons test on Soviet territory to check accuracy of U.S. verification methods. Chervov said the proposal, which would give the Soviet Union the same rights in the United States, had been submitted officially to the United States in meetings of experts of the two countries in Geneva in July.

State Department spokewoman Phyllis Oakley acknowledged that the Soviet proposals for "joint experiments" in the interest of improving nuclear test measurements were presented in a meeting of U.S. and Soviet experts in Geneva from July 13 to July 20.

"Thus far, the sides have not reached agreement for such experiments; our efforts are continuing," Oakley said. She said dates are being discussed for another meeting.