The United States yesterday condemned the Iranian missile attack on a Kuwaiti tanker flying the U.S. flag as "an outrageous act of aggression" against Kuwait, and President Reagan met with top national security advisers to weigh options for an American response.

Initial indications were that the administration is approaching the latest Iranian challenge with extreme caution and consulting closely with U.S. allies in Europe and the Arab gulf states before deciding on any course of action.

Administration spokesmen emphasized yesterday that "the full range of diplomatic and other options" available to the United States is being considered.

While they did not exclude a possible U.S. military strike against Iran, they seemed eager to dampen speculation that such an action is imminent or even necessarily called for in present circumstances.

They noted, for example, that the Iranian missile attack on the Sea Isle City, even if it was a U.S.-flagged tanker, had taken place inside Kuwaiti territorial waters where Kuwait, rather than the United States, was responsible for protecting it.

White House and State Department officials indicated at least another 48 hours would be required for the administration to reach any decision on a response. They also suggested that Reagan's preoccupation with his wife, Nancy, who is to undergo a biopsy today, might delay a decision.

A senior White House official said no meeting between Reagan and his top advisers is scheduled for the weekend and that the administration is in a fact-finding phase.

Before the United States would act, he said, "we have to make sure of all our facts."

Another official said more time is needed for the administration to consult with Kuwait and emphasized that no U.S. military action would be taken without Kuwait's approval.

White House spokesman Marlin Fitzwater said Iran had committed "an outrageous act of aggression against a nonbelligerent country, Kuwait, and a United States-flagged vessel operating commercially and peacefully in the Persian Gulf."

"As in all such cases, we are examining the full range of diplomatic and other options available to the United States," he added.

Fitzwater said Reagan met yesterday morning with his National Security Planning Group, the White House's second highest foreign policy decision-making body, for "a full update" on the attack and that the administration was continuing to analyze the situation.

"The president was brought up to date. There was a discussion of all of these policy implications, but no decisions were made," he said.

He said the administration was not ready to discuss publicly "specific diplomatic or military options at this time."

The president refused to discuss possible retaliation yesterday. Asked how he could allow Iran to "get away" with the attack, Reagan told reporters, "I think maybe you're jumping to conclusions here. I just -- I cannot and will not discuss what our future action may be, but we're in discussions with the government of Kuwait, and it would be very unwise to hint or suggest at anything we might do."

The attack came a day after a Silkworm hit a U.S.-owned, Liberian-flagged supertanker at the same al-Ahmadi oil loading terminal off Kuwait.

In the past three weeks, U.S. forces have attacked an Iranian minelaying ship and four Iranian gunboats.

Reagan held a separate meeting yesterday afternoon with Defense Secretary Caspar W. Weinberger, but both White House and Pentagon officials said it was their regular weekly one and was not provoked by the latest developments in the Persian Gulf.

Some Pentagon officials strongly favor a U.S. military response to the Iranian missile attack. However, one official said Weinberger is not "out of step with the prevailing positions of the administration" that yesterday emphasized caution and consultation before any action would be instituted.

"Everyone is being very careful," the Pentagon official said. "There's not any hipshooting."

Fitzwater and other administration spokesmen went to considerable lengths yesterday to stress that the United States is facing a particularly complex and ambiguous situation because the reflagged Kuwaiti tanker, which has been reregistered in the United States, was in Kuwaiti territorial waters when hit.

U.S. warships, which had escorted the Sea Isle City and three other reflagged Kuwaiti tankers through the gulf earlier this week, have no direct responsibility for them once they reach Kuwait's territorial waters, they noted.

State Department spokeswoman Phyllis Oakley emphasized that the missile strike on the Sea Isle City "was an attack on Kuwait."

"We have extended our protection of the U.S.-flagged vessels in the international waters," she said. "There have been arrangements worked out with the government of Kuwait on when that exact protection begins, but what we're saying {is} in Kuwaiti territorial waters, Kuwait has the responsibility."

Fitzwater also stressed that the attack represented "a somewhat unique situation" because it occurred inside Kuwait's territorial waters.

"Although it was a U.S.-flagged ship, it did not involve U.S. military personnel in any way or U.S. Navy ships," he added.

The administration saw no reason for Congress to invoke the War Powers Resolution because the measure relates only to U.S. military forces, "which were not involved" in the latest incident, he said.

"From a legal standpoint, it does not appear to have an impact on a War Power's Resolution," he said.

Congressional leaders were briefed on the incident at the White House yesterday. Sen. Sam Nunn (D-Ga.), chairman of the Senate Armed Services Committee, said afterward that he regarded the Iranian attack as "a serious escalation" and said Iran should be put on notice that if it uses missiles against American ships it will expose itself to "possible and even probable retaliation."

Meanwhile, the Senate again reserved judgment on invoking the 1973 War Powers Resolution or similar constraints in connection with the U.S. gulf military escort operation, voting 89 to 3 against a proposal to shelve the issue, and setting the stage for a possible showdown next week.

It was by far the strongest showing on a test vote on the war powers issue, although it is unclear whether advocates of congressional intervention of some kind can muster the 60 votes necessary to break the Republican filibuster.

A vote was set Tuesday on a joint proposal to limit debate by Senate Majority Leader Robert C. Byrd (D-W.Va.) and Sen. John W. Warner (R-Va.), ranking Republican on the Armed Services Committee. Sixty votes are required for closure of debate. The Byrd-Warner proposal would require a vote in 90 days that could lead to modification or result in termination of the U.S. military's gulf operation.

Another proposal, sponsored by Sen. Lowell P. Weicker Jr. (R-Conn.), would invoke the 1973 War Powers Resolution under which the operation could not be continued beyond 90 days without congressional authorization. Weicker has failed so far to gain support to force a vote on his resolution.

One alternative to a U.S. military retaliation against Iran reportedly being discussed in the administration is an increase in military assistance to Kuwait to help it cope with the increasing Iranian threat to its territory.

The United States already has sold Kuwait more than $1 billion worth of military equipment, including Improved Hawk antiaircraft missiles and 30 A4 Skyhawk jet aircraft. What kind of aid might help Kuwait to cope with Iran's Silkworm missiles was not immediately clear.

Staff writers Molly Moore, Helen Dewar and Claire Robertson contributed to this report.