SAN FRANCISCO, AUG. 8 -- CIA Director William H. Webster said today he will bring in an outside counsel to investigate the agency's role in the Iran-contra affair and determine whether disciplinary action is necessary against CIA employes.

Webster said reports that he is conducting a housecleaning at the Central Intelligence Agency were "premature," but he plans "to designate a special counsel who was not a part of the agency at the time" to determine whether there were violations of CIA rules or statutes.

Speaking to reporters at the American Bar Association convention here, Webster also said President Reagan's new procedures to inform Congress within 48 hours about most covert operations might have permitted the administration to withhold congressional notice of the Iran arms deal.

"Always in the background . . . is the president's constitutional right to withhold information," Webster said. "Certainly under the agreement, under the constitutional prerogatives it could have been withheld."

But he said the administration is committed to providing the "earliest possible notice" of covert activities, and in most circumstances Congress would be informed before the activity was launched.

Webster said the 48-hour notice provision, contained in a letter Friday from Reagan to the chairman of the Senate intelligence committee, "applies to the finding itself, not the implementation of the finding. It is intended that Congress be notified prior to undertaking an activity in all but the most exigent types of situations."

In "the very few" cases in which Congress is not promptly notified, he said, there will be "an ongoing reevaluation . . . of when it is time to tell, not simply a decision not to tell and putting it on the back burner."

Webster declined to comment on reports about the role in the Iran-contra affair of his predecessor, the late William J. Casey.

"Like every other American who has . . . atched any of it unfold on television, I have my personal views. But I'd rather keep them to myself right now, because they're just speculation," Webster said.

"I've seen a lot of William Caseys portrayed by a lot of different witnesses, and I know that there's very little in a documentary way to support what's been said," he added.

Asked about the circumstances of Casey's death, Webster said he did not "believe that there is any evidence of any conspiratorial activity at work." However, he said, "I suppose that's one more of the great mysteries that will always haunt us."

Webster said increasing U.S. involvement in the Persian Gulf poses a heightened risk of Iranian terrorist activity. But he said it is "more likely it will increase in other parts of the world before it comes to the United States."

In a speech before his news conference, Webster stressed the "essential role" of covert action along with the importance of congressional oversight.

"There must be a trustworthy system of oversight and accountability, which builds, rather than erodes, trust between those who have the intelligence collection responsibility and those who act as surrogates for the American people," he said.

"Having seen some of the responsibilities for covert action move outside the CIA into the National Security Council and, fortunately, removed therefrom never to go back again, we have only added to the confusion, suspicions and the ill-ease of the American people," Webster said.

He also said it is important that the CIA "be seen as giving our best estimates, not to 'cook the books' or to shape or influence policy."