The Soviet Union maintained "a rigid and uncompromising attitude" during the latest round of U.S.-Soviet negotiations on reducing medium-range nuclear missiles in Europe, according to the chief American negotiator, Ambassador Paul H. Nitze.

After making a 30-minute report to President Reagan and his national security affairs adviser, William P. Clark, at the White House, Nitze told reporters that the "Soviet side continued to insist on their one-sided proposal" to keep "a continuing and large deployment of the Soviet SS20s and that the United States not be permitted to deploy any" of its planned Pershing II and ground-launched cruise missiles.

Nitze's description of Moscow's tough stand in the INF medium-range missile talks in Geneva contrasted with more favorable American characterizations of the Soviet position in the START long-range missile negotiations, which are scheduled to continue in Geneva until Aug. 4.

In the START (strategic arms reduction talks) negotiations, according to government officials, the Soviets have put forward proposals for new limits on both sides' nuclear weapon carriers and sub-limits on those capable of delivering multiple warheads. The limits are lower than those in the SALT II treaty, never ratified by the United States.

In addition, the Soviets are reported to have said they would agree to limit the overall number of "nuclear charges"--missile warheads and bombs--on each side. This is the Soviet response, officials said, to Reagan's basic START proposal to limit U.S. and Soviet nuclear warheads.

The Soviet "nuclear charge" proposal was called "vague and unacceptable" by one administration official. Others said the U.S. delegation is trying to get the Soviets to be more precise and to propose sub-limits for nuclear missile warheads separate from nuclear bombs carried by airplanes.

Although the new Soviet proposals have have been described as unsatisfactory by administration officials, they do represent some movement in the START negotiations.

Both sides have now gotten down to defining three common areas for bargaining: delivery systems, including land- and submarine-launched intercontinental missiles and long-range aircraft carrying both bombs and missiles; the number of those systems that can carry more than one warhead or missile, and the total number of warheads or "nuclear charges" on each side.

But in the INF (intermediate-range nuclear forces) negotiations on weapons in Europe, the two sides still have not been able to agree on which weapons to include in the talks.

The Soviets have continued to include British and French nuclear weapons and, according to Nitze, "they also insisted on no limitations whatever on their forces in the eastern portions of the USSR." The United States has insisted that all Soviet missile systems be included to prevent the Soviets from moving their SS20s to the Far East.

A number of U.S. officials said they hold out little hope that an INF agreement can be reached before December, when the United States is scheduled to begin deployment of Pershing II and cruise missiles in West Germany and Britain.

The Soviets have linked the INF and START talks by telling U.S. negotiators that if the new U.S. missiles are deployed, they will have to reassess their START position. The more favorable the Soviets make their START proposal before then, said U.S. officials, the more pressure they put on the Reagan administration to delay the new deployment.

Moscow is also pressing publicly to get the United States to propose the so-called "walk in the woods" formula in the INF negotiations when they resume in September. The formula was sketched a year ago by Nitze in conversations with Soviet negotiator Yuli A. Kvitsinsky when they realized their governments' positions were far apart.

The formula would leave each side with 75 missile launchers in Europe, while the Soviets would limit their missiles in the Far East to 90. It would drastically reduce the Soviet force of about 180 SS20s and allow the United States to deploy only cruise and not Pershing missiles.

The two negotiators agreed to offer the formula to their governments as neither a U.S. nor a Soviet proposal. It initially came as a surprise to the Reagan administration, which nevertheless began working on a modification of it.

The Soviets, however, never sent Nitze a cable acknowledging that the plan was an acceptable basis for compromise. One official in Moscow said later that it was not taken seriously because it was not offered as an official proposal.

Recently, however, Soviet officials have raised the "walk in the woods" formula with West German Chancellor Helmut Kohl and with a delegation of U.S. congressmen.

Administration officials said the Soviets seemed to be saying that the formula could be the basis for discussion if put forward as an American proposal. But the officials said it was conceived as potential final compromise rather than as a U.S. offer to draw a Soviet counterproposal.