The State Department, acting to head off the possible spread of Libyan terrorist activity to the United States, announced yesterday that it has ordered the expulsion of a diplomat attached to the Libyan mission to the United Nations in New York for activities "incompatible with his status and illegal."

The Libyan diplomat was identified as Farhat Tibar, who worked as an administrative attache at the New York mission.

Spokesman Ed Djerejian said the State Department acted after the FBI informed it that Tibar had been linked to "a Libyan-backed plot" to assassinate Libyan dissidents living in the United States.

Djerejian said four incidents had occurred outside the United States so far this year in which opponents of Libyan leader Muammar Qaddafi had either been killed or wounded and Washington was determined to prevent similar assassination attempts from taking place here.

The pattern is clear and we do not intend to let it get to this stage in the United States," he said. "We are doing everything possible to head off this kind of activity here."

Tibar, the first Libyan expelled from the New York mission, was notified late Tuesday he was persona non grata and given 24 hours to leave the country. In 1981, the United States expelled 26 Libyan diplomats and closed the Libyan Embasssy in Washington.

An FBI spokesman refused to comment on whether Tibar's expulsion was linked to the federal grand jury investigation in Alexandria into an alleged plot by Qaddafi operatives here to assassinate at least three Libyan dissidents.

But other federal law enforcement officials confirmed that Tibar was indeed suspected of being involved in this same plot and had been under investigation for some time. The grand jury has subpoenaed 15 to 18 pro-Qaddafi Libyans in Virginia, Colorado, Michigan and North Carolina to give testimony regarding the alleged plot.

No indictments have been returned so far, and five Libyans were granted immunity after they refused to testify and invoked their constitutional right against self-incrimination. They have gone back to the jury for further questioning.

One focus of the FBI probe into the alleged assassination plot has been the Libyan student center in McLean which has been under surveillance for months.

The administration has recently indicated its increasing concern about the possibility that the recent spate of murders of Qaddafi opponents in Western Europe may spread to this country. Since February, three were killed in Europe and the former Libyan ambassador to Austria, now an opponent, was wounded in an assassination attempt in Vienna.

Qaddafi, recently faced with mounting opposition to his regime at home, is reported to have formed groups to go abroad to "chase traitors, fugitives and stray dogs wherever they are and liquidate them physically."