New Zealand Prime Minister David Lange said yesterday that New Zealand will deny a U.S. request for a port call by an American warship next month unless his government knows the ship is not carrying nuclear arms.

A formal letter responding to the U.S. request for a port visit was delivered to the U.S. Embassy in Wellington later yesterday, U.S. officials said. A senior administration official in Washington said it was "not a definitive response," but, he added, "It looks like we are facing a turndown on this request."

In a strong response to Lange's statements, a State Department spokesman yesterday said: "The denial of port access would be a matter of grave concern which goes to the core of our mutual obligations as allies."

If the visit of the U.S. warship is denied, a spokesman said, the United States "would have to reconsider our participation with New Zealand" in an ANZUS naval exercise scheduled for March and "the implications for our overall cooperation with New Zealand" under the ANZUS alliance treaty.

The ANZUS treaty was signed by Australia, New Zealand and the United States as a mutual defense pact more than 30 years ago.

Washington holds that port calls are "essential to the continuing effectiveness of the alliance," a State Department spokesman said.

Since the United States never discloses whether a ship is carrying nuclear arms, a position Australia supports, a ban on nuclear warships in effect precludes port calls by any U.S. Navy vessel.

State Department and Pentagon officials said yesterday there would be no change in that policy.

Speaking at a press conference after two days of talks with U.S. officials in Wellington, Lange referred to the U.S. request and reaffirmed his Labor Party government's policy of refusing to allow port visits by any warships that are nuclear-powered or armed with atomic weapons.

"This is going to be our continuing policy. It is not anti-American, it is not anti-alliance, it is anti-nuclear," he said, in a brief statement before the press conference.

"If there was a possibility of verifying that nuclear-capable ships did come to New Zealand with nuclear arms on board, then of course such ships would not be acceptable," Lange said, United Press International reported from Wellington. "If we don't know whether they are nuclear armed or not, then they can't come," said Lange.

The prime minister swept to power in elections last July, and his antinuclear stand was a major campaign issue.

A New Zealand decision to deny port calls by U.S. warships would represent a significant security problem in the alliance, Pentagon and State Department officials said.

Such a decision could also add fuel to a growing antinuclear lobby in Australia, whose calls for the removal of three key U.S. communications bases and opposition to port calls by nuclear-armed or nuclear-powered U.S. warships is of concern to the ruling Labor Party government of Prime Minister Robert Hawke and top U.S. policy makers.

In addition, such a move could raise concern that other allies in Asia and the Pacific region could also ban U.S. warships from making port calls.

A senior State Department official said the New Zealand reply "still leaves the door open a crack." He said U.S. officials are in contact with Lange and are trying to obtain a "definitive" response.

Even if Washington is unsuccessful in this particular request, he said, the United States will keep trying with other requests and hopes to have the issue resolved before the annual ANZUS meeting in July.

If it is not resolved, he said, the United States will be facing "some tough decisions this summer."

"I don't know how it's possible to have a security relationship with an ally that wants help in mutual security but says your ships can't visit," said one Pentagon official.

The issue is also expected to be discussed when Australian Prime Minister Hawke comes to Washington next weeek.

If the port visit is denied, it is not likely that next month's naval exercise, called Sea Eagle 1-85, would be able to proceed with New Zealand's participation, Pentagon and State Department officials said.

New Zealand has said it intends to participate in the annual naval exercises despite its policy on port visits.