President Reagan's national security affairs adviser, Robert C. McFarlane, said last night that the United States "must be free to consider an armed strike against terrorists or those who support them" when there is reason to believe that a terrorist attack cannot be prevented by other means.

"We cannot and will not refrain from preemptive actions where conditions warrant," McFarlane said in a speech to the National Strategy Information Center, a private organization concerned with national security issues.

In reiterating administration willingness to use force against terrorism, McFarlane was following the line taken by Secretary of State George P. Shultz. He added that for "a measured dose" of counter-terrorist force to be effective, "we must be able to act quickly" and "we cannot be bogged down in interminable consultations and debates."

McFarlane cited as a possible case where preemptive action or a retaliatory strike might be justified the connection between terrorist attacks on Americans in Lebanon by Shiite Moslems and their incitement by Iran.

"We need not insist on absolute evidence," he said about cases in which there are strong grounds for suspecting a connection. "Nor should we need to prove beyond all reasonable doubt the tie between terrorist actions and its supporters.

"To renounce force as an option is to invite more terrorism," he said. "Where other remedies are inadequate, force must be available. There is no real debate in this administration on that basic principle. There should be none among free people who wish to remain so."

In addition to citing Iran, McFarlane recalled that when the Reagan administration first took office, it "pointed a finger unequivocably at the Soviet Union" as the sponsor of several terrorist groups. He added: "There has been no reason to modify those statements."