The United States has undertaken delicate diplomatic soundings with an ally, Turkey, and an adversary, the Soviet Union, aimed at obtaining three-cornered approval of U2 reconnaissance flights important to the verification of the strategic arms limitation treaty (SALT II).

The talks were intended to be highly secret because of the political sensitivity of the subject in both Ankara and Moscow, but press reports from those two capitals brought the essence of the maneuvering into public view. American officials are unhappy and apprehensive about the leaks because the discussions are incomplete, with no clear sign yet whether they will succeed or fail.

The unusual situation illustrates the extent to which the SALT II treaty is affected by domestic as well as international politics in far-flung capitals. It also is a dramatic indication of how times have changed since the 1950s, when it was unthinkable that a NATO ally would insist on Soviet acquiescence in reconnaissance flights intended to observe the Russian homeland and equally unthinkable that the United States would present the matter of its spy flights to the Soviet Union with hope of success.

The maneuvering was triggered by the loss of U.S. ground intelligence stations in Iran that closely monitored Soviet missile tests across the border. Although the United States has several other means to check on Soviet missiles in flight, the Iranian bases were particularly important in recording the early stages of a test launch and listening to electronic data, or telemetry, sent back to earth.

U2 reconnaissance flights just outside the Soviet border over Turkish air space are considered the best quickly available way to recoup some of the capabilities lost by the shutdown of the Iranian stations. Because of the angle and altitude of observation, such U2 flights are believed to be more effective in this respect than the American satellites that monitor the Soviet Union from space.

The flights over Turkey are also believed to be more effective for some aspects of monitoring Soviet missiles than the U.S.-operated stations on the ground in Turkey. These stations were closed on Ankara's orders for 3 1/2 years in retaliation for the congressionally imposed arms embargo against Turkey resulting from the Greek-Turish conflict in Cyprus. The Turkish stations were reopened last October after the United States resumed arms sales to Turkey.

President Carter, faced with senatorial and national debate over the U.S. ability to monitor SALT II agreements in great detail, is reported to have sent a secret letter early last month to Turkish Prime Minister Bulent Ecevit asking for approval of the proposed U2 flights as a contribution to world peace and East-West detente.

Ecevit, according to a detailed account in the Turkish independent newspaper Milliyet, said in reply that he recognized the importance of the flights but asked for "firm assurances" that the Soviet Union would not object to them.

As Turkish relations with the United States became difficult in the past several years due to the arms embargo by its NATO ally, Turkish relations with the Soviet Union improved substantially. Ecevit signed a friendship treaty in Moscow last June. Just this week, in the latest sign of close relations, the two nations signed a pact calling for Russian aid to Turkey's oil and natural gas explorations.

The United States pursued the verification flights matter in a visit to Ankara early this month by Deputy Secretary of State Warren Christopher. He is reported to have emphasized the potentially important contribution of the U2 flights to SALT and the potentially serious effect of their rejection on U.S.-Turkish relations.

Christopher is reported to have promised to give Turkey a written document on the purpose of the flights and the Soviet attitude. According to press reports from Moscow, which are not denied by informed officials, U.S. soundings aimed at Soviet acquiescence have begun.

Administration officials said they had no information to confirm an ABC news report last night that the Soviets are "prepared" to give tacit approval to the U2 flights over Turkey.

There is no doubt that the discussions come at a sensitive time for all concerned. The United States is facing a ratification debate on SALT, Washington and Moscow are preparing for the U.S.-Soviet summit meeting next month, and Congress is in the midst of considering arms aid to the Turkish ally.