If the Carter administration eventually moves direct military pressure against Iran, informed sources say, the most likely tactic will be use of mines dropped by U.S. Navy jets to seal off ports from which Iran exports oil.

These sources say such a strategy would effectively block Iranian oil from getting out by sea while not interfering with tankers loading up at other Persian Gulf ports in Kuwait, Saudi Arabia, Qatar, Oman and the United Arab Emirates.

It also has the advantage, sources say, of not requiring an actual naval blockade or guarantine of the entrance to the Gulf, an operation that would require even larger naval forces than the United States already has on the scene and could entail risks of direct confrontation with other countries, possibly including the Soviet Union.

The United States has aerial mines stored aboard a task force that has been steaming in the Arabian Sea for months and that now included a detachment of 1,800 Marines. The force also includes two aircraft carriers with about 150 jet warplanes.

President Carter, though not ruling out eventual use of force in the hostage crisis, has made it clear that any military option would be invoked only if all else fails.

Administration officials say military options such as the mining plan still are, in fact, on the "back burner." The current administration effort is aimed at creating wider pressure against Tehran by trying to convince allies in Europe and Japan to join the United States in imposing economic and diplomatic sanctions against Iran.

However, if that effort -- and possibly still more unilateral nonmilitary U.S. moves -- fails, then some military measures may be taken and the mining scheme is said to be the most likely at this point. Officials add there are other military plans that could be used if the situation changes dramatically.

The appeal of the aerial mine laying plan derives from the concentration of Iran's oil export facilities in the northeastern tip of the gulf. The main oil spigot where international tankers fillup is located at Kharg Island. Thus, the mission could be carried out quickly in a concentrated area and not involve direct military attack.

Nevertheless, the plan undoubtedly would run grave risks for the safety of the hostages.

The U.S. Navy, using similar tactics, mined the main North Vietnamese harbor of Haiphong in May 1972, in a pivotal action during the final phases of the war in Southeast Asia.

In mid-January, The Washington Post reported the Navy was developing contingency plans to blockade or mine portions of the gulf and that President Carter had discussed both the blockade and mining options with Defense Secretary Harold Brown and other advisers.

Those plans have been kept in the drawer. But they have now been refined and, according to both informed civilian and military sources, they are now viewed as the most likely tactic if the current economic and diplomatic efforts fail and some military action is called for.