On the eve of another session of the United Nations Lawof the sea Conference, American representative Elliot L. Richardson fired a warning shot yesterday across the bows of other participating nations.

The chief of the U.S. delegation, who has been pessimistic publicly about the ability of the 158 nations ever to agree on a treaty-urged participants in the four-week session that begins here this week not to endanger the "filigree-like character" of agree-ready.

In addition to his warning, however, Richardson said that the United States was encouraged by the results of the last session, which recessed in May in Geneva.

"It may be said one day that the Geneva phase was the turning point in the life of the conference," Richardson told a press conference.

The obstacles to success remain formidable, Richardson added. Last spring he put the odds against success at 3 to 1. He declined to set new odds yesterday, but said they seemed some what improved.

The decade-old conference is aimed at governing access to the oceans and seabeds that cover 72 percent of earth. It is generally acknowledged to be of be of vital importance to global development, but it is just as generally ignored by all but a few experts who have followed its painfully slow progress.

A main sticking point has been how the world will mine the riches that lie on the seabed. Nodules of nickel, manganse, cobalt, copper and other minerals lying at depths up to 15,000 feet may be worth trillions of dollars.

While the law of the Sea Conference is dedicated to the principle that resources under the sea are the "common heritage of mankind," it is clear that some nations' ideas of how to exploit that heritage have little in common with the ideas of others.

Many poorer nations insist on an international agency to mine the minerals for all. Industrial nations, led by the United States, oppose giving total control to an international body.

The United States, as Richardson reiterated yesterday, insists that its access to the seabed minerals be guaranteed. The United States now imports almost all the cobalt and managenese it needs, 90 percent of its nickel and about 20 percent of its copper. Exploitation of the seabed could make the United States self-sufficient in these minerals.

Other nations have been alarmed by congressional action on legislation that would allow U.S. companies to mine the seabed in the 1980s.

Richardson said that the final bill is not likely to permit such mining until at least 1983, and that it is imperative to have some interim regulation of mining until an international treaty is negotiated.

While trying to allay the fears of those who see the legislation as an American attempt to exploit the "common heritage" without regard for our nations. Richardson said he believes the legislation is needed quickly.

He took issue with the director of the Office of Management and Budget, James T. McIntyre, Jr. who said passage of the seabed mining bill this year was not critical.

Richardson said that in the absence of certainty about how and when mining would be allowed companies with the technical ability to exploit the seabed were being discouraged from making the large preparatory investments needed.

He said he belives there is a growing realization at the United Nations that the U.S. legislation is not a threat to a seabed treaty which would supersede it, and that there is aneed for an interim legal framework.