The U.S. Marines in Lebanon, who until now have been largely isolated in the Beirut International Airport, will expand their duties to begin "limited" patrols of the main thoroughfares in predominantly Christian East Beirut, the Reagan administration announced yesterday.

State Department spokesman John Hughes said the 1,200 Marines were ordered by President Reagan to join French and Italian troops in the 3,800-man multinational force in undertaking the new assignment at the request of Lebanese President Amin Gemayel.

Hughes added that these new duties will involve only part of the U.S. contingent at any one time and will not require any increase in its size.

Pentagon planners have indicated that the force, which entered Beirut Sept. 25 after the massacre of Palestinian civilians there, may be given increasingly broader peace-keeping duties throughout Lebanon to facilitate a withdrawal of Israeli, Syrian and Palestine Liberation Organization forces there.

However, Reagan administration officials, sensitive to the potentially controversial political implications of a widened American role in Lebanon, sought to stress the limited nature of the new assignment announced yesterday.

Hughes noted that the multinational force "has the mandate to facilitate the assertion of the government of Lebanon of its authority over all of Beirut. The government of Lebanon requested the participation of the U.S. in this exercise which is part of its effort to restore the control of the central government over all parts of the capital city."

Pentagon officials added that the patrols were intended to be "a politically symbolic expression of international backing for Gemayel." The aim, they said, is to show that the Gemayel government has control over all the city.

In the U.S. view, these officials stressed, there is no significant risk involved in these new duties for the Marines because East Beirut, in contrast to the Moslem western half of the city, has a predominantly Christian population supportive of Gemayel, who is Christian, and has been almost totally free of violent incidents since the Israeli siege of the city ended.

According to the officials, the Marines will operate under the same rules that have applied to their airport duties. While they will be authorized to defend themselves if attacked, they will not have actual police functions. Instead, their presence is intended to reinforce the authority of Lebanese army and police units.

Hughes said motor patrols would be dispatched along the main boulevards of East Beirut. Pentagon officials added that, while final details have not been worked out, the Marines and French and Italian troops are expected to work in close liaison with Lebanese units or possibly even in "mixed patrols" of Lebanese and multinational troops operating together but traveling in separate vehicles.

Since negotiations began to get the Israeli, Syrian and PLO forces out of Lebanon, there has been persistent speculation that the multinational force might have to be expanded and redeployed to monitor the Beirut-Damascus highway during disengagement of Israeli and Syrian troops, help to maintain a security zone along Israel's northern border in southern Lebanon and patrol areas of eastern and northern Lebanon where there are heavy concentrations of Palestinians.

However, U.S. officials, noting that the negotiations are still in a very early stage, have insisted that the future role of the multinational force cannot be determined until there is an agreement on withdrawal that will make clearer what arrangements are required and what functions are beyond the capability of the Lebanese forces to perform.