U.S. and Soviet officials are offering different previews of this week's meeting between the superpowers' leaders, with the Americans trying to dampen expectations raised by the Soviet promise of a "Christmas gift to the American people and mankind."

"We have a president who is leaving office in a few weeks, who is not going to be making any promises that he is not here to keep," said White House spokesman Marlin Fitzwater."And we have a new president who will be taking office in a few weeks, who is not going to be making commitments when he doesn't have an administration in power that can fully analyze and make these judgments," he said.

Fitzwater commented as both sides prepared for the meeting of President Reagan and President-elect George Bush with Soviet leader Mikhail Gorbachev on Wednesday in New York City.

The session will be held after Gor-bachev addresses the U.N. General Assembly. Secretary of State George P. Shultz and Soviet Foreign Minister Eduard A. Shevardnadze will meet in the morning at the U.S. mission to the United Nations.

Gorbachev, due to arrive later Tuesday from Moscow, is expected to make wide-ranging proposals to the General Assembly and possibly in his meeting with Reagan and Bush.

A top Soviet Communist Party official, Nikolai Shishlin, said Sunday that Gorbachev would have "a Christmas gift to the American people and to mankind" on his visit to New York.

The Reagan administration, however, continued to play down the event.

Fitzwater said that while Gorba-chev could spring a surprise, it would not be "embarrassing" if the meeting concluded without a breakthrough or an agreement.

Fitzwater also suggested that U.S. officials would not let Gorbachev put them under pressure to respond immediately to some new initiative.

"We are being very realistic," Fitz-water said. "We find nothing embarrassing or in any way negative about characterizing this meeting as a friendly discussion, an open airing of issues between us and a healthy approach towards continuity."

He said that simply by meeting, the U.S. and Soviet officials illustrate "the strength of the new (Washington-Moscow) relationship."

Bush, meanwhile, insisted that he would participate in the luncheon only as vice president. But, he added: "I expect they'll be aware they're talking to the next president."

Echoing that line, Senate Minority leader Bob Dole, R-Kan., said Reagan is "going to be in charge until the 20th of January. George Bush understands that, Gorbachev understands that. But (Gorbachev) also knows that's not very far away."

Under diplomatic protocol, Bush will be at the meeting as Reagan's understudy, not as president-elect.