President Reagan said yesterday that the seven Americans kidnaped in Lebanon in the last 16 months "are being moved around quite often" and that their lives could be endangered by precipitous U.S. action.

" . . . We must be very careful and not precipitate that threat being carried out," Reagan said in response to a question at a luncheon with out-of-town broadcasters.

Reagan's statement came on a day when a senior administration official said that Syrian President Hafez Assad was the "one person" who could obtain the release of the seven Americans, and when FBI Director William H. Webster warned that "we must keep our cool" in dealing with terrorists.

Speaking to 100 corporation lawyers here for the American Bar Association convention, Webster said the United States "must be willing to accept efforts at reprisals" by terrorist groups if action is taken against them. He said that any U.S. action must be directed against "known culprits" and that the United States must be able to "justify to the world" its response.

The cautious tone of Reagan's and Webster's remarks contrasted sharply with the president's militant speech to the ABA convention on Monday when he accused Iran, Libya, North Korea, Cuba and Nicaragua of forming an international terrorist network "engaged in acts of war" against the United States and declared U.S. willingness to act "unilaterally, if necessary, to ensure that terrorists have no sanctuary anywhere."

Asked a second time what the administration is doing to free the kidnaped Americans, Reagan said: "The situation is, how do you rescue someone if you don't know where they are? How do you rescue them if, at the same time, you would have to reveal yourselves to the point that you might be bringing home a body rather than a human being?"

At this point, U.S. efforts to free the Americans seem to be pinned largely on Assad, who played a leading role in freeing the 39 Americans held hostage in Lebanon from Trans World Airlines Flight 847. Speaking of Assad, the senior U.S. official said, "He's not totally in control of the situation, but he's the one person who can obtain their release."

David Kimche, director general of Israel's Foreign Ministry, told The Washington Post last week that the Americans are thought to be in Lebanon's Bekaa Valley, which is "completely under the domination of the Syrian army and even more so under the domination of Syrian intelligence."