NATO countries have agreed in principle to the new negotiating stance the United States will propose at the Geneva talks on medium-range missiles, alliance sources said here today. The revised position reportedly includes a demand for equality of deployed missiles in Europe--rather than globally--in exchange for a Soviet guarantee that its forces in Asia will remain at their current level.

The reported agreement came after U.S. Assistant Secretary of State Richard Burt secretly briefed NATO allies in the high-level Special Consultative Group here yesterday. That clears the way for announcement of the revised U.S. proposal by Washington.

President Reagan is scheduled to address the United Nations General Assembly on Monday.

The reported change in the U.S. position, which was confirmed by U.S. officials in Washington, amounts to a significant shift. It means that the United States will allow the Soviets to keep the missiles they now have in Asia. In the past, the United States has demanded that these also be counted in any quest for equality between the two superpowers.

Officials in Washington describe this as an indication of administration sincerity in the search for an arms control agreement.

Today's assent in principle by the alliance follows several weeks of reports about new U.S. proposals, including a letter from President Reagan to the European allies last week on the compromises the United States is prepared to make in the talks, which resumed in Geneva for their most crucial round on Sept. 6. U.S.-made Pershing II and cruise missiles will begin to be deployed in Western Europe in December unless U.S. and Soviet negotiators reach agreement.

The revised U.S. proposal also comes as the European allies prepare for a potentially disruptive autumn with antinuclear activists planning major protests in an effort to halt the deployment.

Washington's planned insistence on equality only in Europe, where the Soviets reportedly have 250 of the highly mobile SS20s, in exchange for a Soviet pledge to keep their forces in Asia at their current level--324 warheads on 108 medium-range SS20s--addresses a particular concern of Japan.

The United States is known to have engaged in intensive arms control discussions with Tokyo in recent weeks as the administration moved toward presenting a new position in Geneva, and it is believed that Washington has sought to combine movement at Geneva with assurances to Japan that its security interests will be protected.

More than a year ago, the chief U.S. and Soviet negotiators in Geneva informally worked out the so-called "walk-in-the-woods" formula, under which each side would be left with 75 missile launchers in Europe and the Soviet missile force in the Far East would be restricted to 90 SS20s. The plan would have canceled deployment of the U.S. Pershing IIs, which can strike Soviet territory from West Germany in less than 10 minutes, and the Soviets would have dismantled most of their SS20s. Both Moscow and Washington rejected the controversial formula, but in recent months there have been calls to examine the plan again.

On Aug. 27, Andropov proposed to "liquidate" Soviet medium-range missiles in the European part of the Soviet Union--including a "considerable number" of triple-warhead SS20s--in excess of the 162 comparable missiles deployed by Britain and France. He tied his proposal to U.S. renunciation of the planned December deployment. The Soviet leader reiterated that offer today

Elimination of the Pershing IIs is not thought to be included in the revised U.S. negotiating stance.

Another aspect of the new U.S. position is that the medium-range aircraft capable of carrying nuclear weapons would be counted in the Geneva negotiations, NATO sources said. The two sides are far apart on which aircraft to include.

The current American position calls for eventual destruction of all intermediate-range missiles but in the interim allows the United States to deploy as many of such weapons as Moscow maintains.