The Reagan administration, responding to pleas from U.S. mining interests, has decided to block early completion of the nearly finished Law of the Sea Treaty involving 150 nations and seven years of negotiation.

The administration's action was formulated at an interagency meeting Monday chaired by Deputy Secretary of State William C. Clark, and made public in a little noticed one-paragraph press statement.

The statement referred to "serious problems" in the current draft of the lengthy and complex treaty, which contains 320 clauses. The State Department yesterday identified these problems as the deep seabed mining provisions, especially a desire to ensure the access of United States industry to seabed minerals on "fair and reasonable" terms.

Washington's decision is likely to generate a strong reaction next week as delegations from throughout the world gather at the United Nations for what had been expected to be the final six-week round of the marathon global negotiations.

Singapore's Ambassador T.T.B. Koh, who has been a leader in the talks, said he was extremely upset to learn of the decision from U.S. diplomats, especially now that it is too late to head off the forthcoming negotiation meetings involving more than 150 countries. Koh expressed the fear that the U.S. posture will undercut compromises proposed or approved by the Nixon, Ford and Carter administrations and backed successfully by "moderate delegations" of the Third World.

The U.S. delegation to next week's negotiation, according to the State Department announcement, has been instructed to make sure that the treaty is not completed pending a policy review in Washington. The upshot of the review may be new U.S. positions on seabed mining that would require reopening previously negotiated sections. It is uncertain whether the rest of the nations of the world will agree to such a bid.

The most important opposition to the current draft of the treaty, according to those who have followed the matter, comes from major corporations that are heavily involved in plans or programs of deep seabed exploration. These include corporate combines headed by Lockheed Aircraft Corp., United States Steel Corp., and Kennecott Copper Corp., respectively.

Last July's national platform of the Repubican Party, responding in part to pleas from the mining interests, charged that the Law of the Sea negotiations "have served to inhibit U.S. exploration of the seabed" while "concern has been lavished" on Third World nations. Subsequently members of Congress from both parties have criticized the treaty draft in letters to President Reagan.

Monday's interagency meeting is reported to have featured shifts in position by several U.S. agencies. For example, the Defense Department previously had been among the most powerful backers of an overall Law of the Sea agreement because of provisions protecting the right of passage in sea lanes. At this week's meeting, according to informed sources, the representative of the Joint Chiefs of Staff continued to back the treaty but the secretary of defense position, expounded by Undersecretary-designate Fred C. Ikle, was that "serious problems" involving seabed mining must be resolved.

Hearings on the Law of the Sea negotiations have been scheduled for Thursday before a Senate Foreign Relations subcommittee headed by Sen. Larry Pressler (R-S.Dak.) In a letter to the president, Pressler said ratification of the current version of the treaty would encounter "great difficulty" in the Senate because of objections regarding seabed mining.