Maxwell M. Rabb, U.S. ambassador to Italy, has been in Washington for more than a week, in part because of a "security threat" against him, State Department officials said yesterday.

Attention was focused on Rabb's presence here by an article in today's editions of The New York Times. It said Rabb had been flown to the United States hastily from Milan last Wednesday after Italian authorities discovered a Libyan plot to assassinate him.

However, State Department sources, while confirming they had reports of a threat against Rabb, said the principal reasons for his return were a combination of personal business and use of his assistance in lobbying members of Congress for approval of President Reagan's proposed $8.5 billion sale of Airborne Warning and Control System (AWACS) planes and other aircraft equipment to Saudi Arabia.

Despite the report that Rabb left Italy on Wednesday, he is known to have been in Washington for more than a week. On Oct. 15, he lunched with Sen. Larry Pressler (R-S.D.) just before the Senate Foreign Relations Committee voted to disapprove the AWACS sale.

In its editions of Oct. 19, Newsweek reported that Libyan leader Muammar Qaddafi had sent a hit team to Italy to assassinate Rabb in retaliation for the U.S. downing of two Libyan jets over the Gulf of Sidra last August.

The Newsweek report was the object of extensive attention in the Italian press, and the sources said yesterday they believed this was the basis for the current rumors.

Although they refused to discuss the subject of Libyan involvement, the sources did say that the reports of danger to Rabb were being pursued by Italian authorities, and that his trip here was arranged partly as a precautionary measure while this investigation is under way.

However, the sources here continued, there had been no direct threat to Rabb, and they insisted that he had not left Italy precipitously.