Central Intelligence Director Stansfield Turner has told the General Accounting Office that the proposed $12 billion sale to Iran of seven highly sophisticated airborne radar systems might jeopardize the securtity of the United States.

Turner's reservations were expressed in a letter ot GAO which is part of a still-classified report that concludes that, apart from political considerations the sale of the radar systems to Iran is not justified, sources said.

The proposed sale, announced secretly to Congress by the Carter administration late last month, has drawn a storm of protest from Congress. Motions to block the sale will be filed in both the House and the Senate, congressional sources said.

The GAO report of which Turner's letter is a part was requested by Sen. John Culver (D-lowa), a leading Senate opponent of the sale. It is expected to form the basis of congressional opposition to the sale when hearings on the motions are held next week.

In response to a question from GAO. the sources said, Turner wrote that the potential for an all-Iranian crew to defect to the Soviet Union with the radar system, a techonology the Soviets do not now possess could constitute security problems for this country.

A CIA spokesman yesterday refused to elaborate on Turner's opinion cited in the GAO report.

The full report the sources said concludes that the Boeing. and Westinghouse-produced radar system known as the Advanced Warning and Control System (AWACS). was too limited to serve Iran's needs, and that "an adequate search for other alternatives was not made."

The report does not address the question of whether preferable alternative systems exist, the sources said.

Meanwhile, Culver yesterday released the text of a "Dear Colleague" letter he and nine other senators have sent to the full Senate, arguing that the potential compromising of new technology and the new arms sale policy Carter announced in May make the proposed sale unwise.

"President Carter's policy is that the United States will not be the first supplier to introduce into a region new or significantly higher combat capability, reads Culver's letter. "It would be a serious mistake to set a precedent by making this sale to Iran a major exception."

Culver said he will introduce a motion Friday to block the proposed sale. Rep. Gerry E. Studds (D.-Mass) has already introduced a similar resolution in the House. Both must be approved by Aug. 6 if the sale is to be blocked.

The revelation of Turner's opinion of the AWACS sale my touch off a new round of controversy within the intelligence community. Sources said analyses by intelligence agencies in the Defense and State departments concluded that the risk of AWACS falling into Russian hands is minimal.

The sources said others in the intelligence community are angered that Turner did not coordinate his response to GAO with them, and view his maverick opinion as designed to further his stated goal of consolidating intelligence operations under one man.

The system at issue is a modified Boeing 727 jet, with a circular plate of the most advanced radar and communications equipment manufactured by Westinghouse mounted stop it. The plane the Iranians want to purchase would not have all of the advanced devices the U.S. planes carry.

Estimates of AWACS' cost vary from source to source. The cost per unit as around $50 million if research and development costs are not factored in, sources said, but if development costs are included the unit cost soars to well over $100 million, making AWACS the most expensive plane in production in history.

When hearings on the AWACS sale open before the Senate International Relations Committee next week, opponents are expected to use Culver's arguments, and to add that selling AWACS to Iran would require more American technicians - the GAO report estimates 400. sources said - in Iran.

Supporters of the sale. which was initiated during the Ford administration and carried toward culmination under Carter, will argue that AWACS is less lethal than other American weapons the shah wants but has been refused, and that the United States should not risk alienating an important Persian Gulf ally.

In a related matter sources yesterday confirmed a report that there exists an internal Navy memo that indicates the Defense Department prevented Iran feom seeing a Grumman Aircraft Corp. plane that is a less expensive and complex version of AWACS.

The source said. though that other Iranian officials including Vice Minister of War Gen. Hassan Toufanian had seen the Grumman plane and chose AWACS instead.