The United States and the Soviet Union have reached "essential agreement" that Secretary of State George P. Shultz and Foreign Minister Andrei A. Gromyko will meet in January to begin a new phase of arms-control discussions, administration sources said last night.

The sources said a series of confidential talks between U.S. and Soviet diplomats, mostly in Washington, has cleared the way for the high-level meeting, which may be announced in the two capitals later this week.

The breakthrough was first disclosed last night by NBC News diplomatic correspondent Marvin Kalb, who reported that the Shultz-Gromyko session will take place in Geneva at the end of the first week in January.

A meeting between the two is considered a first step toward the possible resumption of full-scale negotiations on reduction of nuclear arms, talks that have been in limbo since late last year. However, the two nations have sharply different views on what should be negotiated, with the Soviets giving priority to stopping the development of weapons in space, and the United States reluctant to consider agreements in this area.

There was no indication that the agreement to talk has closed or even narrowed the substantive differences between Washington and Moscow. There was also no indication that U.S. officials have resolved their differences over the details of the American positions in future talks.

The private diplomatic meetings on the date, place and agenda of the Shultz-Gromyko talks followed a series of public and private statements in which leaders of both nations expressed their wish to improve relations and work on ways to slow the spiraling arms race.

The most recent top-level message came last Saturday, when the Soviet Union expressed willingness to work out details of a Shultz-Gromyko meeting to resume the superpower dialogue.

The Soviet message was signed by President Konstantin U. Chernenko, according to administration officials. This fact was first reported yesterday in The Boston Globe by the newspaper's diplomatic correspondent, William Beecher.

After the election, President Reagan sent a message to Chernenko expressing his commitment to improved relations and arms reductions in his second term, officials said. But it was unclear whether Chernenko's message was a direct response to Reagan's, according to the sources.

To facilitate discussions with the Soviets across a broad arms-control agenda, the administration is considering appointment of a "special envoy" or "special coordinator."

Two prominent figures, retired general Brent Scowcroft, who led the presidential commission on the MX missile, and Paul H. Nitze, who was U.S. chief negotiator with the Soviets on medium-range missiles in Europe, have been mentioned in official circles as possible appointees to such a new position.

The White House informally sounded out Scowcroft in recent weeks, but no agreement was reached on conditions of such an appointment, according to informed sources. They said Nitze was sounded out late last year for the job of negotiator on all nuclear-arms questions when and if U.S.-Soviet bargaining resumed, but the White House was not willing to promote him to the job over Gen. Edward L. Rowny, who has led the U.S. delegation to the strategic arms talks.

The administration already has an arms-control coordinator in Kenneth L. Adelman, director of the Arms Control and Disarmament Agency, but he is thought to lack the specialized knowledge for detailed discussions with the Soviets.

Adelman, in an article soon to be published in Foreign Affairs magazine, is proposing "arms control without agreements," in which the two sides would adopt parallel restraints on weapons in areas where full-scale treaties cannot be achieved. Adelman said this approach is his idea but that his article was cleared by higher-ups who encouraged him to state his views.

State Department officials rejected a report that the department has adopted the Adelman idea in interagency bargaining.