In the most explicit signal yet that the administration is poised for a possible shift in its position in talks with the Soviet Union on nuclear arms in Europe, U.S. negotiator Paul H. Nitze said today that the United States "is certainly not locked into the zero-option" proposal.

Arriving here after consulting with allies in Brussels and Bonn, Nitze said in a prepared airport statement that President Reagan "has made it clear that we are prepared to examine every proposal" that meets the security interests of the West.

When pressed on possible U.S. flexibility, the veteran negotiator brushed aside most questions except for underscoring in precise terms not previously used by top-ranking U.S. officials that there is room to maneuver away from the "zero option."

The Geneva negotiations between the United States and the Soviet Union to limit medium-range nuclear missiles in Europe, are scheduled to resume Thursday after a two-month recess.

Nitze's remarks indicated that the United States now recognizes publicly that a final agreement could not be obtained with the Soviet Union on the basis of Reagan's zero option that called for the Soviets to dismantle their arsenal of intermediate nuclear missiles if the North Atlantic Treaty Organization cancels deployment of 572 Pershing II and cruise missiles in Europe beginning in December.

In his State of the Union message Tuesday night, President Reagan reiterated that the United States is "prepared to carefully explore serious Soviet proposals." Hinting at room for compromise, the president said, "We insist on an equal balance of forces," without specifying at what level that balance must be.

Emphasizing that any final deal with the Soviets "must be equitable," Nitze said that the current impasse in the arms talks demonstrated that "the issues are complex and not susceptible to easy solutions."

His Soviet counterpart Yuli Kvitsinsky also arrived here today and told waiting reporters that the issue of controlling nuclear arms is "urgent and acute".

Kvitsinsky said the "time is approaching when the choice will have to be made on joint steps" if a compromise is to be reached in the coming months.

After saying that the new Soviet Communist Party leader, Yuri Andropov, had offered "far-reaching proposals" last month for the Soviets to reduce their number of missiles to the 162 deployed by France and Britain, Kvitsinsky called on the United States to respond with its own ideas in the arms negotiations.

"If the other side is willing" to reach an arms agreement, he said, "that willingness should be demonstrated."

In recent weeks, a crescendo of European pleas for the United States to adopt a more flexible position in the Geneva negotiations has reflected mounting anxiety among many governments that growing public opposition may make deployment of the NATO missiles exceedingly difficult.

In his discussions with NATO ambassadors in Brussels and Bonn government officials, Nitze heard more suggestions that the West should seek to settle for an interim accord that would restrict the number of missiles stationed on both sides in a transitional phase aiming toward ultimate elimination of all intermediate-range nuclear missiles.

After staunchly supporting the U.S. position at the beginning of the negotiations, the British government has started to encourage a partial agreement that would limit missile deployment, if only to contain the increasingly hostile demonstrations against the NATO plans to install the modern nuclear rockets.

The West German government of Chancellor Helmut Kohl, seeking to lure voters before March 6 elections in which the missiles have emerged as a key issue, has spoken with a divided voice on the zero option, but desires progress in the negotiations to reassure voters that the United States is making a sincere effort to reach an arms control accord.