BEIRUT--Gunmen blocked a southern highway yesterday and abducted a top-ranking U.S. Marine officer serving with the U.N. peacekeeping force, bringing to nine the number of Americans held hostage in Lebanon.

U.N. and Pentagon officials identified the victim as Lt. Col. William R. Higgins, 43, of Danville, Ky., chief of an observer group attached to the U.N. Interim Force in Lebanon (UNIFIL).

The U.N. group, comprised of 76 officers from 16 countries, monitors cease-fire violations on the Lebanon-Israel border. Higgins was the top-ranking American officer assigned to the group, holding the title of senior U.S. military observer.

There was no immediate claim of responsibility for the abduction, which took place at about 4:15 p.m. on the coastal highway between Lebanon's southernmost port of Tyre and Naqoura, a town on the Israeli border that is UNIFIL headquarters.

"Higgins was driving in a U.N. jeep station wagon from Tyre to Naqoura behind a similar vehicle in which two other observers were traveling," said U.N. spokesman Timur Goksel.

He said the first vehicle went round a bend in the road, "and when the two observers noticed that Higgins' car was not following them, they stopped and drove back, only to find Higgins' station wagon abandoned."

UNIFIL helicopters and ground troops are searching for Higgins, who was alone in his vehicle, Goksel said.

Another American and a Dane were in the other vehicle, but UNIFIL officials did not name them.

Security sources in Tyre said two gunmen in a brown Volvo passed Higgins' car, blocked the road, forced Higgins into their car and drove north toward Tyre.

They said the abduction occurred between Ras el-Ein and Deir Qanoun, villages controlled by Justice Minister Nabih Berri's Shiite Amal militia. U.N. sources said Amal was helping UNIFIL search for Higgins.

Reporters in Tyre say the influence of the Iranian-backed Hezbollah, or Party of God, has been growing in the villages. Hezbollah is believed to be the umbrella group for radical Shiite Moslem factions holding most of the foreign hostages in Lebanon.

Twenty-four other foreigners are captives in Lebanon, including eight Americans. Held longest is Terry A. Anderson, chief Middle East correspondent of The Associated Press, kidnapped March 16, 1985.

Higgins was returning from a meeting in Tyre with Abdel Majid Saleh, an Amal political leader in the area, Saleh told reporters. Efforts to free foreign hostages were among their topics, he said.

One source said privately the kidnappers took Higgins to Shabriha, a village half a mile northeast of Tyre.

President Reagan was informed at his California ranch and told reporters as he left for Washington he was "trying to learn more about it."