The United States has asked the Soviet Union to open formal talks on adding on-site verification provisions to an eight-year-old underground nuclear test ban treaty, according to informed government sources.

The proposal was made Feb. 17. Although the State Department sent a follow-up inquiry to the Soviets last Monday, no response had been received by yesterday, according to these sources.

"We had expected some form of a reply by now," one American official said.

The administration is seeking an indication of how flexible Soviet leader Yuri V. Andropov will be on arms control matters.

Sources on Capitol Hill, in the State Department and inside the Arms Control and Disarmament Agency voiced concern last week that failure of the Russians to respond might eventually lead to a resumption of unlimited underground testing.

The Reagan administration is asking the Soviets to reopen talks on a treaty signed in 1974 that limited both sides to tests of 150 kilotons or less. The United States and the Soviet Union say they have been abiding by the treaty since 1976, even though it has not been ratified.

A refusal by Moscow to reopen discussions, sources said, could lead to renunciation of the pact by either side and a resumption of testing of new generations of high-yield nuclear weapons. That possibility had been raised and set aside during interagency consideration of the administration's plan, according to sources.

The dilemma is, as another official put it yesterday: "What do we do if the Russians don't agree to talk?"

The American proposal, which called for technicians from both countries to examine a new U.S. plan for on-site inspections, was delivered to a Soviet diplomat, Minister-Counselor Oleg M. Sokolov, in Washington on Feb. 17 by Assistant Secretary of State Richard Burt and James George, acting director of the Arms Control and Disarmament Agency.

Soviet Embassy officials last week refused to comment on the matter.

The treaty originally was signed by then-President Nixon but has never officially gone into effect because neither the Nixon, Ford nor Carter administration pushed for ratification by the U.S. Senate.

As drafted, the threshold treaty contains no provisions authorizing on-site verification of the explosive power of tests. At the time it was drafted, and during the ensuing years, American administrations were satisfied that existing seismic and satellite systems could determine satisfactorily whether the Soviets had cheated on the limit.

Last July, President Reagan decided not to resume talks among the United States, Great Britain and the Soviets on a comprehensive nuclear test ban, the next step in controlling nuclear tests. Instead, it was announced that the United States would initially seek revision of the threshold treaty because of allegations that the Soviets had exceeded the limit on some tests in recent years.

Last year, Radio Moscow termed this Reagan action "part of an extremely dangerous policy of preparing for a nuclear war."

Government officials were deeply divided over the move to reopen the threshold treaty. It took more than six months, and pressure from Sen. Charles H. Percy (R-Ill.), for the administration to decide what it would do about the threshold treaty. Percy, chairman of the Senate Foreign Relations Committee, threatened to hold up a vote on the controversial nomination of Kenneth L. Adelman as director of ACDA if he did not get action on the treaty.

In the ensuing bureaucratic debate, officials at the Department of Energy, which builds and tests nuclear weapons, came out for ratification of the existing agreement. One of their arguments was that, even if the Soviets were exceeding the 150-kiloton limit by 100 percent, it made no militarily significant difference. This is because a 300-kiloton test wouldn't reveal any more information about how a megaton weapon would work than would a smaller test. Megaton weapons, those with the power of more than 1 million tons of TNT, are of prime concern to military strategists.

Another group, primarily in the Department of Energy and the nuclear weapons laboratories, fears the Soviets will accept the proposal with the result that "Russians would be crawling all over the Nevada test site," as one weapons builder put it recently.

The position that eventually won was promoted primarily by the Defense Department. Defense argued that, if the Russians cannot be persuaded to adopt the on-site verification proposals planned for the monitoring of the threshold treaty, "more intrusive ones needed for other treaties could never be obtained," in the words of one Pentagon official.

The new American plan would provide for technicians of either side to be present at tests of 75 kilotons or more. That figure was chosen, according to sources, because some scientists say that current measurements are up to 100 percent inaccurate in establishing the explosive power of such tests.

According to sources, the government plans to use a portable electronics unit developed by the Los Alamos National Laboratory. The device measures the size of the explosion through a cable attached to the buried container holding the nuclear device.

The shock waves from the explosion send a pulse to an instrument above ground, permitting an accurate measurement within 10 minutes after the explosion.

In opposing the administration's decision to push for on-site verification, some weapons builders argued that the Soviets would not have a device similar to the Los Alamos one, and would attempt to use a more intrusive method in monitoring the American tests. "They the Soviets ," one scientist said, "have always wanted to get a look inside one of our weapons tests."

One official suggested last week that in order to keep the Soviets away from American nuclear tests, the weapons laboratories would lower the yields of their tests to below 75 kilotons.

"If they the Soviets do the same," one official said, "the end result of the Reagan initiative would be to ratchet down the testing levels and not have on-site inspections on either side."