The two Americans among four military officers killed in Lebanon Saturday when their jeep struck a mine had been moved into the Beirut area a few weeks ago as part of a strengthened United Nations observer force, sources said yesterday.

Army Maj. Randall A. Carlson, 35, of the Montgomery County community of Derwood and Air Force Maj. Harley S. Warren, 34, of Valdese, N.C., were part of an observer force that began service in Lebanon when hostilities ended in the Middle East after Israel gained independence in 1948.

The truce observers' role is to monitor possible cease-fire violations, and 35 Americans are serving with such units in Egypt, Jordan, Syria and Lebanon.

Carlson and Warren had been with observer units in southern Lebanon until the Beirut observer force was increased from five to 30 persons a few weeks ago, according to U.S sources. The others killed in the explosion nine miles east of Beirut were an Irish major and a Finnish captain.

Carlson's last letter to his brother Donald, a congressional legislative aide who lives in Derwood, spoke of the future, of returning home, of possible teaching jobs, of further study, of settling down after a hard tour of duty in the war-torn Middle East.

"In his latest letters, Randy expressed concern that he was traveling daily in parts of Lebanon that were controlled by Israel, Lebanon or the Christian militia," Donald Carlson, 33, said Saturday. "It was his birthday just this week.

"He was a dedicated Army officer," Carlson said. "His favorite assignment was teaching . . . . He was hoping that, after a year in this Mideast assignment, he would go to one of the Army staff colleges."