President Reagan and Soviet leader Mikhail Gorbachev have agreed on an approach to limiting long-range, or strategic, nuclear weapons that may require far more intrusive on-site inspections than accepted under the precedent-setting Intermediate-range Nuclear Forces (INF) Treaty, U.S. officials said yesterday.

The approach, outlined in the statement issued by the two leaders at the conclusion of their summit meeting in Washington on Thursday, calls for on-site inspections at locations where either side suspects the other of cheating. It also calls for development of procedures for verifying compliance with treaty limits that could require on-site inspection of operational nuclear weapons.

The INF Treaty, signed by Reagan and Gorbachev Tuesday afternoon, only allows field inspections of medium-range and shorter-range weapons that are to be dismantled and destroyed, not retained in U.S. and Soviet arsenals. It also restricts inspections of suspected cheating to a limited number of sites agreed upon in advance.

"The inspection procedures under a strategic arms treaty will have to be a lot tougher than those under INF," a senior U.S. official said yesterday. "Under the INF Treaty, we will eliminate whole categories of weapons, which makes verification easier because there aren't any weapons left to count."

The two sides agreed at the summit to allow 4,900 warheads apiece on land- and sea-based strategic missiles, a substantial cut from current levels. By contrast, the INF Treaty applies to 2,611 weapons.

In addition, a proposed agreement on long-range arms would allow both sides to retain a specified number of weapons, each posing a greater and more direct risk to U.S. security. This will greatly complicate the task of ensuring that weapons have not been hidden away by the other side, in secret readiness for war, the official said.

At the summit, the leaders agreed to allow either side the right to conduct on-site inspections at a moment's notice "at locations where either side considers covert deployment, production, storage or repair of strategic offensive arms to be occurring."

Working groups of U.S. and Soviet officials did not settle on the type or number of sites where these inspections could be conducted. But U.S. officials said both sides recognized the need to permit more widespread inspections than allowed under the INF Treaty.

Under one U.S. approach, the two sides would permit inspection of facilities they agree look like possible weapons sites.

The tentative formula for reducing strategic warheads could require U.S. inspections of Soviet submarine-based missiles on board ship, U.S. officials said, or Soviet inspections of U.S. missile silos in Wyoming and North Dakota.

The formula, proposed by the Defense Department, calls for all missiles of a specific type, such as the U.S. Trident II, to be counted as having the same number of warheads. The number would be set by the United States and verified by the Soviets through direct on-site inspection.

The formula departs from the approach taken under SALT II, the 1979 strategic arms agreement that was never ratified by the Senate. Under SALT II, any missile was assumed to have the maximum number of warheads with which it was flight-tested.

This did not require on-site inspections because the number of warheads used in missile tests can be readily observed by satellites and distant radars.

The Pentagon is resisting the SALT II approach because it wants to gain new flexibility in its planning by testing missiles such as the Trident II with more warheads than it will initially deploy on them.

Some lawmakers and independent arms control experts have said the Pentagon formula is unverifiable, because the U.S.S.R. could secretly add more warheads to its missiles after any inspection.

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, has complained that "although on-site inspection . . . could help, nothing could dissuade the Soviet Union from adding more reentry vehicles during {a} time of crisis."

The senior U.S. official responded yesterday that although the Soviets "might be able to do this in individual cases, they could not quickly reload all of their missiles in a massive breakout of an arms control regime."