A partial text of President Reagan's identical letters to Senate President Pro Tem Thurmond and House Speaker O'Neill:

On Sept. 29, 1982, I reported to you concerning the introduction of United States armed forces in Lebanon to participate in the multinational force (MNF) requested by the government of Lebanon.

The presence of this force was designed to facilitate the restoration of Lebanese government sovereignty and authority and thereby further the efforts of the government of Lebanon to assure the safety of persons in the area and bring to an end the violence . . . .

I directed this deployment pursuant to my constitutional authority with respect to the conduct of foreign relations and as commander-in-chief . . . .

We have periodically provided Congress with updated information on the activities of these forces and on the circumstances of their deployment . . . . In light of recent events, I am providing this further report . . . in accordance with my desire that Congress continue to be informed on this matter and consistent with Section 4 of the War Powers Resolution.

On Aug. 28, sporadic fighting between Lebanese armed forces and various armed factions took place in south Beirut; from time to time . . . positions in the vicinity of the Beirut airport manned by U.S. Marines . . . came under small-arms fire (without injury to U.S. personnel), and this fire was returned.

On Aug. 29 . . . Marine positions came under mortar, rocket and small-arms fire, with the result that two Marines were killed and 14 wounded. In addition, several artillery rounds fell near the USS Iwo Jima (an amphibious support vessel lying offshore), with no resulting damage or injuries.

As contemplated by their rules of engagement, U.S. Marines returned fire with artillery, small arms and, in one instance, rocket fire from a helicopter gunship. There were additional exchanges of fire earlier today, Aug. 30, without injury to U.S. personnel.

Later today, a cease-fire came into effect in the Marines' area. . . and firing on Marine positions ceased. Diplomatic efforts are under way to extend this cease-fire. In the meantime, U.S. forces will be prepared to exercise their right of self-defense . . . .

I believe that the continued presence of these U.S. forces in Lebanon is essential to the objective of helping to restore the territorial integrity, sovereignty and political independence of Lebanon.

It is still not possible to predict the duration of the presence of these forces in Lebanon; we will continue to assess this question in the light of progress toward this objective.

I will keep the Congress informed as to further developments . . . .