The State Department yesterday warned Americans working for a Libyan civil airline against becoming involved in military operations such as Libya's occupation of Chad.

The warning was issued by State Department spokesman Alan Romberg, who said the department had "unconfirmed reports" that U.S. crewmen of Libya's United African Airways have participated in resupply flights into Chad.

Romberg said 10 to 20 American pilots and mechanics work for the Libyan airline, primarily a charter outfit. American officials have sometimes referred to it as Libyan leader Muammar Qaddafi's "Air America," a reference to the supposedly private American airline that worked for the Central Intelligence Agency in the 1960s.

The spokesman said the department has "no indications" that any U.S. citizens are serving in or with the Libyan air force. Earlier this week, The New York Times reported that American pilots and mechanics were flying and maintaining Libyan air force planes.

Referring to the reports of American citizens' involvement in the resupply of Libyan forces in Chad, Romberg said it is "totally inappropriate for any U.S. citizen to take any action which helps Qaddafi carry out such illegal activities."

He added, "We advise any U.S. citizens undertaking such activities to cease." He did not say what might be done if the "advice" is not taken, saying this is a question for the Department of Justice.

While repeating again in public the State Department recommendation that Americans not visit or reside in Libya, Romberg declined to describe continuing U.S. oil firm operations in Libya as actions that "help" Qaddafi carry out illegal activity. This indicated that there is no change in the Reagan administration's stand that the oil operations should be discouraged but not forbidden.

On Sept. 24, in a little-noticed action arising from the reports that Libya has used U.S. civil aircraft in Chad, the Commerce Department tightened controls on the servicing of Libyan planes. Under the new rules, a separate export license is required every time a Libyan aircraft is serviced with parts or equipment originating in the United States. Previously, the planes could be routinely serviced under "bulk licensing" arrangements.

In announcing the change, the Commerce Department said Libya is "known" to have used U.S. civil aircraft in its military operations in Uganda and, more recently, in Chad.

Officials said some of the Libyan civil aircraft were bought with valid licenses from U.S. manufacturers, and thus are subject to American-imposed restrictions on their use. The officials said some other U.S. aircraft were acquired on the international black market, and others were bought from U.S. manufacturers before official restrictions were imposed.