Authoritative sources confirm that U.S. intelligence has received a very detailed--although in some respects puzzling--report about a 10-man squad allegedly formed to assassinate President Reagan or his top Cabinet officers, including Secretary of State Alexander M. Haig Jr. and Defense Secretary Caspar W. Weinberger.

The report is understood to provide the name of each squad member and known aliases used by each in the past. It is said to include details on where the men were trained and reports that some of that training was in Eastern Europe.

All except perhaps one or two members of the squad are said to be Libyans.

The reports that Libya has sent such a team into the United States are being taken seriously but nevertheless are a source of puzzlement within the global U.S. intelligence and security network.

The source or sources for information in the intelligence report is said to be described only vaguely in the report.

While it would be standard procedure to provide only vague references to sourcing in order to protect the informant or informants, in this case the vagueness is part of the problem in evaluating the information and has caused doubts about its accuracy.

The doubts are summarized as follows:

Although Libyan ruler Col. Muammar Qaddafi is viewed as a dangerous and unpredictable leader, some analysts doubt that he would put his name to an assassination plan which, whether it were to succeed or be exposed in failure, could lead to an incendiary aftermath, including a U.S. military attack on Libya.

Similarly, if such an assassination plan actually were in effect, it likely would be a most closely guarded secret, and the ability of an informant to obtain the kind of detailed information on each squad member, as is now circulating, is viewed as highly unlikely.

Furthermore, a 10-man team is viewed by some specialists as too large, offering too great a chance for slip-ups by one or two members.

There also is some doubt about reports that team members were trained in Eastern Europe. This refers to the volatility of the mission and the feeling that no nation in Eastern Europe would take a chance on being associated with it. On the other hand, Qaddafi's internal security service is trained and run by East Germans.

Sources stressed that despite these questions, the report is being taken seriously.

As to the source of the information, the possibilities are that the information is accurate; that it was so-called disinformation, deliberately meant to be inflammatory for some unknown purpose, or that somebody wanted to make money out of a situation in which such information would seem plausible and valuable.

In its current issue, Newsweek magazine reports that the principal informant defected in Western Europe three weeks ago and told U.S. officials he was present about two months ago in Libya at a meeting during which Qaddafi issued an order to kill Reagan or, failing that, other ranking U.S. officials.

Newsweek said the defector is in CIA custody in the United States and was unknown to U.S. officials before telephoning a U.S. diplomatic mission to seek asylum and money.

It is believed that the closest watch on Libyans trying to enter the United States centers on the Canadian border, the longest and easist to cross into the United States, and on Switzerland, by reputation a place where it is somewhat easier to obtain a visa to the United States.

Although the administration expelled Libyan diplomats last summer, Libyans are still being allowed into the United States.