Terrorists could be planning an attack on a passenger ship in the eastern Mediterranean Sea, or an aircraft in Europe or the Middle East, the State Department said Friday night.

A statement from the department's deputy spokesman, Richard Boucher, said "the risk of acts of international terrorism in Europe and the Middle East is increasing."He said the department had received "specific and credible" information about a threat to ships in the eastern Mediterranean, and added, "Such an operation may be undertaken at any time."

"We note that civil aviation remains a particular target," Boucher said.

The department issued its warning at the unusual hour of after 8 p.m. on Friday in part because,"It's important that we share the threat information with the American people as they plan their travel," said another spokesman, Adam Shub.

Terrorists have attacked ships before. In December 1985, a group led by Abu Abbas hijacked the Italian cruise ship Achille Lauro as it left an Egyptian port, holding it for two days before surrendering to Egyptian authorities.

One of the hijackers shot to death an American passenger, Leon Klinghoffer, who was confined to a wheelchair.

A particular reason for concern Friday was threats by the Abu Nidal Organization, which "has repeatedly threatened to attack U.S. interests" if Mahmoud Atta, a 37-year-old U.S. citizen and member of the organization, is extradited to Israel, the department said.

Atta, who also uses the name Mah-moud El-Abed Ahmad, is wanted in Israel on charges of participating in the April 1986 machine gun attack on a bus carrying Jewish settlers in the occupied West Bank. The driver was killed and a passenger wounded.

Arrested in the United States three years ago, Atta fought extradition in the courts but lost his last appeal in the U.S. Supreme Court on Aug. 31. The department has two months from that date to decide to turn him over to Israel, and Friday night Shub repeated earlier statements that it would do so.

"The United States takes these threats very seriously in view of the ANO's notorious history," Boucher said.

Shub said the Nidal group was believed responsible for more than 90 terrorist attacks since 1974. Using the cover name "Black September," it arranged the September 1970 hijacking of four airliners to an abandoned airfield in Jordan, he said.

Among its attacks were bombings of the airports in Vienna and Rome in December 1985, the hijacking of a Pan Am airliner to Karachi in 1986 and the attack on the day excursion vessel City of Poros in Greece in July 1986, the department said.