In another move flowing from its opposition to the international Law of the Sea treaty, the Reagan administration is preparing to claim minerals in the sea within 200 miles of U.S. shores, informed sources said yesterday.

A presidential proclamation creating the 200-mile zone will be announced at the State Department, probably today, they said. The United States already asserts control over fishing rights within 200 miles.

The U.S. move comes only a week before nearly all the other nations of the world are to meet at Kingston, Jamaica, to discuss implementation of the treaty, which was worked out with the help of the Ford and Carter administrations, but which the Reagan administration refused to sign.

Among those expected to attend the Jamaica conference are U.S. allies who have signed the treaty, including Canada, France, Japan and the Netherlands, and U.S. allies who so far have not signed, including Belgium, Britain, Italy and West Germany. Some 120 nations have signed the document, far more than enough to put it into force when formal ratification documents are received.

The politics of the situation, as explained by experts in the field, is that the Reagan administration is attempting to show by this action that a nation can control the seabed resources independent of the treaty, which it refuses to sign.

A draft of the proposed proclamation, circulated among the allies more than six weeks ago, reportedly departs to some extent from the provisions of the Law of the Sea treaty. Diplomatic sources said this was questioned by Canada, a strong supporter of the treaty. Other countries, including New Zealand, let the United States know they hope that the U.S. decree would not depart from the Law of the Sea provisions.

Lee Kimball, executive director of Citizens for Ocean Law, which supports the Law of the Sea treaty, called the forthcoming proclamation "a unilateral action not in U.S. interest at this time." She said any inconsistency between the proclamation and the provisions of the treaty could provoke other nations to make claims detrimental to U.S. interests.

State Department officials dealing with the issue declined comment.