U.S. arms control negotiator Paul H. Nitze said yesterday that he is "confident that if it becomes wise for the United States government to change its position" in talks with the Soviet Union on reducing nuclear missiles in Europe "it will in fact do so."

Nitze gave no indication that the Reagan administration is considering any shift, but his remarks in an NBC television interview suggest for the first time that there may be some flexibility in the U.S. position at the Geneva talks if the Soviet Union offers a better proposal than it has.

Nitze said he was "completely satisfied" with the backing he has received from the Reagan administration and with the U.S. negotiating position.

In an earlier interview yesterday with The Washington Post, Nitze also said that some ideas he and his Soviet counterpart discussed informally last year about how to reduce nuclear missiles in Europe "was not an agreement" in the sense of an offer by either negotiator authorized by his government.

Referring to the Soviet negotiator, Nitze said that "what Ambassador Yuli Kvitsinsky and I developed was an exploratory package" of ideas "which was not an offer on behalf of the United States and not an offer by him on behalf of the Soviet Union, but which we considered both governments might wish to look at."

The Soviets, Nitze said, "firmly rejected" the package, while Washington "didn't feel they were adequate" but did not shut the door as clearly as did Moscow.

Nitze, who rarely makes public statements on his role as the chief U.S. negotiator at the Geneva talks on limiting intermediate-range nuclear forces (INF) in Europe, did so yesterday in response to a storm of controversy here about the negotiations.

Administration officials are known to be concerned that recent shake-ups in the U.S. arms control agency, news accounts about what has gone on behind closed doors in Geneva, an aggressive campaign of speeches by Soviet leaders and continued pressure on the White House from here and abroad to reach an accommodation with Moscow all threaten to undermine their ability to manage these difficult negotiations.

In Bonn, West Germany, yesterday, Soviet Foreign Minister Andrei A. Gromyko also rejected reports that the U.S. and Soviet negotiators had earlier reached any tentative agreements at Geneva. Gromyko acknowledged that informal contacts had taken place but said "there was absolutely no truth to the rumors" that any breakthrough had been achieved.

Nitze, who will meet with President Reagan on Thursday or Friday to get his final instructions before returning to Geneva, is certain to play a crucial role if any agreement is to be achieved with Moscow.

It has been known for many months that Nitze and Kvitsinsky held informal talks last July about finding common ground between opposing U.S. positions. In recent days, however, some accounts suggested that informal agreements were reached between the two men, which Washington, as well as Moscow, then disowned.

Officials are concerned that this, too, will make the administration look as though it is not interested in arms control and that some shift in policy has taken place, which officials insist is not the case. Administration officials acknowledge that the two ambassadors did put together a package of ideas, but say that this exploratory effort has been blown way out of proportion.

U.S. officials say they believe Moscow still feels that by working on public opinion in western Europe, especially, it can stop the intended U.S. deployment of 572 new nuclear missiles there without paying any price.

The officials say they believe the West must remain firm behind Reagan's proposals for as long as it takes to convince Moscow that the missiles will be deployed unless the Soviets are more forthcoming at the negotiating table. Reagan would forgo the U.S. missile deployment if Moscow dismantles some 600 of its missiles already in place, an idea Moscow rejects.

Officials have suggested, as Nitze did yesterday, that there eventually may be some American compromise. But they say they believe it is necessary first, for example, to see the results of the March elections in West Germany. A victory there by the current conservative government, which supports Reagan's approach, presumably would make Moscow more willing to offer better compromises than it has offered thus far.

In an effort to explain his situation and presumably maintain his credibility as a negotiator, Nitze said yesterday, "I have had in the past, and expect to have now, the authority to probe and explore whether or not there is any possibility for a major change in the Soviet position."

Nitze said he did not have the authority now or in the past to propose agreements that would deviate from Reagan's policy. The idea, officials said, is for Nitze to probe the Soviets for maneuvering room and bring back any signs of movement to Washington for evaluation.

"The president's initial instructions to me were to negotiate seriously and to examine carefully any serious alternative to the zero solution that the Soviets would propose," Nitze said in his interview with The Post, adding that he was sure Reagan was willing to consider any such alternative. It was under those instructions that Nitze believed it proper to put together ideas informally with Kvitsinsky.

Reacting to reports that he was rebuked for this by the White House, Nitze said, "I was not reprimanded."

"I have continued to support and believe in the basic U.S. position," Nitze said, "that the Soviet Union, Europe and the world would be better off if both sides were to forgo missiles of this class."

Behind the revival of interest in Nitze's informal talks with Kvitsinsky is Reagan's recent firing of Nitze's former boss, Eugene V. Rostow, as director of the U.S. Arms Control and Disarmament Agency.

Some senior officials believe Rostow and his aides are now making too much of the informal talks to bolster Rostow's reputation as one who pushed for breaking the deadlock on arms control only to be fired, at least in part, for his efforts.

These officials said that is not the case. They said Rostow did support Nitze's efforts but that Nitze was the prime mover and that Rostow's role is now being overstated.