Nations, N.Y. — No promises, no threats.
This is how analysts here describe the policy now adopted by the Reagan administration toward the Law of the Sea Conference.
Following recommendations made by the National Security Council last weekend, the President decided that the United States would attend the new round of talks of the Law of the Sea Conference starting March 10 in New York. The US intends to seek major changes in agreements that have so far been reached, but would not present the conference, as had recently been feared, with an ultimatum.
''It will not be a matter of take it or leave it,'' says a US diplomat.
The softening of its tactical stance by the US toward the conference was greeted at United Nations headquarters with a sigh of relief.
''The worst has now been avoided: a Law of the Sea Treaty is unlikely to be signed by all UN member states without the participation of the United States,'' says one well-placed expert.
According to reliable sources, the tactical shift of the Reagan administration vis-a-vis the conference was determined by:
* Its isolation, when it became clear that its allies would not turn their back on the conference.
* The realization at the highest level of government that the treaty contained many extremely positive elements for the US.
Diplomats who have participated in the Law of the Sea negotiations for years realize that ''the United States is not yet on board'' and that, in fact, the stance it has now adopted may prolong the talks and serve to postpone the signing of a treaty.
But they also say that a further delay in reaching a final agreement is preferable to an outright confrontation with the US. ''This administration has already come a long way in learning how to live with the real world, and we must give it more time to adjust to a complex reality,'' says a senior Western expert on the Law of the Sea.
Last March the US slammed the door at the Law of the Sea Conference on the very day when it was convening in New York: It announced that it would not attend the meeting and needed time to review its position.
One hundred fifty nations including the US had worked during eight years and had come close to an agreement on the numerous aspects of the sea law, including the setting of territorial limits, free passage through straits, and the mining of potato-size manganese nodules containing copper, cobalt, and nickel.
Only last month the administration indicated that it would take a very tough position with the Law of the Sea Conference and walk out of it if major changes in some agreements were not made. Presumably, a US consortium called Ocean Minerals Company ) would then go it alone and start mining the ocean floor in 1988 under existing US laws.
Now as then, the US is expected to present the conference with a list of ''problems.'' It is expected to insist, among other things, on having a permanent seat on the Law of the Sea Council, the decisionmaking organ.
''Prospects for a treaty to be signed this year are now dim. There will be much haggling and arm twisting. But the conference is already behind schedule, and another year's delay is a price worth paying for getting the US back into the fold,'' says a longtime watcher of the Law of the Sea debate.