Iran has begun laying antiship mines in the northern Persian Gulf and the United States is assessing the potential threat to U.S. Navy ships and Kuwaiti tankers sailing under American flags, a senior administration official said yesterday.

The official said marine salvaging sources reported that the mines have been laid in waters leading into Kuwait's main oil port at Ahmadi.

At first, they were thought to have broken lose from the Shatt al Arab waterway marking the Iran-Iraq border, but "It now looks as if these may be mines that have been deliberately planted out there," the official said. "This is a new development. It's not clear how much of a threat it's going to be."

The possible use of mines -- allowing Iran to sabotage Kuwait-bound ships but avoid a direct military confrontation with the United States -- appears to be a contingency the administration has yet to fully consider. At least one Soviet tanker, leased to Kuwait earlier this year, hit a mine in mid-May just outside Kuwaiti waters.

The Defense Department is planning to add three more heavily armed warships to the five-vessel U.S. Middle East Force under its plan to provide a military escort for 11 Kuwaiti ships that are being reregistered under the American flag.

Pentagon spokesman Robert B. Sims said Tuesday there are no plans to add mine sweepers to the fleet. He said the Defense Department is still evaluating the potential threat of the mines.

Sims said there are "other ways" to deal with the threat than sending U.S. mine sweepers. One U.S. official noted that Saudi Arabia has four U.S.-built mine sweepers in its small navy.

The Soviet Union, which has leased three oil tankers to Kuwait, now has four mine sweepers in the Persian Gulf and is adding a guided missile cruiser, according to Assistant Secretary of Defense Richard L. Armitage, who testified at the Senate Foreign Relations Committee Tuesday.

The use of mines is a tricky issue because the Iranians appear to be laying them only inside Kuwaiti waters where U.S. Navy ships escorting the Kuwaiti tankers are not now scheduled to enter. Thus, the Kuwaiti tankers could be successfully escorted through the gulf only to be in danger of later striking Iranian mines.

This is not the first time the United States and its Middle East allies have contended with a mine threat. In the late summer of 1984, a Libyan ship spread mines in the Red Sea in an attempt to sabotage traffic passing through the Suez Canal.

The United States, along with a number of other western powers, sent mine sweepers to destroy what turned out to be makeshift Libyan mines that did no damage.

Sharp congressional criticism of the administration's plan to reflag the 11 Kuwaiti tankers continued yesterday.

Rep. Les Aspin (D-Wis.), the chairman of the House Armed Services Committee, said the Central Intelligence Agency had told his panel that its analysis rated as "quite high" the likelihood of a "no-fingerprints" Iranian retaliation, "such as a terrorist attack against Americans or increased attacks on non-U.S. flag shipping."

Aspin also suggested that Iran might try to use mines, not in the Strait of Hormuz where the current is too swift, but elsewhere, "say in the channel entering Kuwait's harbor."

Aspin said that many in the U.S. intelligence community did not share "the bland assessment" in Defense Secretary Caspar W. Weinberger's report to Congress Monday on military risks from Iran to the U.S. naval escort in the gulf.

He said there are "immense" differences in the administration over assessment of the risks and that he plans to hold hearings, possibly starting Friday, to air the differing views of the CIA, Defense Intelligence Agency and State Department. Aspin also said he is planning "a series of speeches" in the House addressing the whole issue of U.S. policies in the Persian Gulf.

A Pentagon spokesman disputed Aspin's charge that the administration is divided on the threat to shipping.

Sen. James Sasser (D-Tenn.), one of three senators who toured the gulf after the May 17 attack on the USS Stark, suggested that the administration should ask Kuwait to withdraw its reflagging request.

The U.S. reflagging is "unnecessary and unwise," Sasser said in a report to Congress.

Senate Foreign Relations Committee Chairman Claiborne Pell (D-R.I.) said yesterday he has decided to postpone until next Thursday a markup on his bill prohibiting the United States from going ahead with the reflagging and calling instead for creation of a U.N. peacekeeping force in the gulf.

An aide said Pell felt that sentiment on his bill, or an alternate, had "not ripened to a point where a vote could be held tomorrow."Staff writers Helen Dewar and George C. Wilson contributed to this report.