President Reagan said yesterday that he takes seriously and is "concerned" about a U.S. intelligence report that a five-person Libyan assassination squad has entered the United States on a mission to kill him or his top advisers.

Acting after an informant provided U.S. officials with what one intelligence source called an unconfirmed warning of an assassination plot, the president ordered Secret Service protection Thursday for his three top White House advisers: chief of staff James A. Baker III, counselor Edwin Meese III and deputy chief of staff Michael K. Deaver.

Extra security precautions have also been in effect in recent weeks for the protection of the president, Vice President Bush, Secretary of State Alexander M. Haig Jr. and Secretary of Defense Caspar W. Weinberger, after earlier reported threats that they were marked for assassination by terrorists trained in Libya.

Now, informed sources said, agents of the Secret Service and FBI are working throughout the country to try to track down an alleged assassination squad that was said by one informant to have entered the United States this past weekend with orders to kill the president or his advisers.

" . . . there is a threat to them that has been made rather obvious," Reagan said yesterday in confirming he has ordered security protection for his top aides.

He added, "Obviously you'd have to be concerned about . . . all the people that have been named in this."

Asked if he takes the threat seriously, Reagan responded, "I think you have to. I think it safe to say that in any security case, even when security gets what they think is a crank call, they can't take that for granted."

Intelligence sources familiar with the case said that within the last few days an informant who is not an American has given authorities the names of five people who he claimed were members of a Libyan assassination squad. The informant also reportedly claimed to have been involved in the planning for the assassination effort. But intelligence sources stressed that the credibility of the informant has not been ascertained nor has his story been verified.

Another intelligence source added, "We frankly don't know whether it is fact or fiction. But we can't afford to think in terms of how well he the informant is to be trusted. We have got to think in terms of it can't be dispelled, so we have got to act."

The New York Times first reported yesterday that an informant had told U.S. authorities about a five-person Libyan assassination team that allegedly entered the United States last weekend.

Last night, ABC News reported that U.S. intelligence officials have "partially identified, with names and pictures," some of those believed to be the Libyan agents.

At the White House, where memories of the March 30 shooting of the president and the critical wounding of press secretary James S. Brady are still painfully fresh, the matter is being treated seriously.

The press office yesterday issued a statement saying that members of the president's staff had met with and telephoned representatives of major news organizations to request that the media exercise "restraint in reporting and televising specific security measures utilized in the protection of the president and others."

Washington editors and managers for The New York Times, Washington Post, ABC, CBS and NBC said their organizations would act with restraint in reporting security measures.

Federal investigators are reviewing case files on two former U.S. intelligence agents who have been indicted for providing terrorist equipment and training to Libya in order to track down former military and intelligence agents who have worked in Libya, according to one federal official.

This source said the case files of indicted fugitives Edwin P. Wilson and Francis E. Terpil are not being reviewed because authorities have linked them with the alleged assassination team that is said to have entered the United States.

Rather, he said, federal officials are seeking all available information on sources who might be able to provide details of terrorist or assassin training in Libya.

The Libyan news agency Jana, monitored in Beirut by Reuter news agency, charged last night that the Reagan administration was disseminating a "series of lies" in commenting on the reports that a Libyan-trained assassination team had landed in the United States.

"The American claims and series of lies to which the American administration resorts in its foreign policy stem basically from hatred and intolerance, and clearly reflect the American terrorist line against the people of the Libyan Jamahiriyah," the agency said.

United Press International, meanwhile, carried an unconfirmed report that Lebanese security sources said Libyan gunmen planned to assassinate Reagan's special envoy, Philip Habib, during his current visit to the Mideast. But Lebanese Prime Minister Shafiq al-Wazzan was quoted by Reuter as saying yesterday he had received no such reports from security sources.