BEIRUT, FEB. 17 -- A U.S. Marine officer in charge of a U.N. observer group in Lebanon was kidnaped by unknown gunmen today as he drove in a two-car convoy near the southern port city of Tyre.

The officer, identified by Pentagon officials as Lt. Col. William Richard Higgins, 43, of Woodbridge, Va., was seized after leaving a meeting in Tyre with Abdel Majeed Saleh, a senior political official of the mainstream Shiite Moslem Amal group, Amal officials said.

By late tonight there was no claim of responsibility for the attack and no indication of who might have carried it out or of the motive.

The kidnaping, which brings to nine the number of Americans held captive in Lebanon, brought immediate expressions of concern from the White House and the United Nations, whose peace-keeping forces in southern Lebanon mounted a wide search for the missing officer.

"Most of {the U.N. peace-keeping force} is involved in the search, including helicopters, and we are getting substantial help from Amal," Timor Goksel, spokesman for the U.N. peace-keeping force, said.

The White House called for the prompt release of Higgins. President Reagan, asked about the kidnaping as he boarded Air Force One in California to return to Washington after a vacation, said, "We're still investigating, trying to learn more about it."

U.N. Secretary General Javier Perez de Cuellar, traveling in Africa, expressed "profound concern," a U.N. spokesman in New York said, and Undersecretary General Marrack Goulding, who is traveling in the Middle East, began diplomatic efforts to gain Higgins' release.

The kidnaping came just 12 days after two Scandinavian officials of the U.N. Relief and Works Agency, which administers the Palestinian refugee camps in the Middle East, were kidnaped by unknown gunmen in southern Lebanon, and it raised new fears for the safety of the thousands of foreigners working in various U.N. organizations in chaotic Lebanon.

Higgins served with the U.N. Truce Supervision Organization, a small unit that, since its formation in 1948, has supervised armistice agreements between Israel and neighboring Arab countries. The organization's 75-man unit in Lebanon, which Higgins headed, worked closely with the much bigger -- and better known -- U.N. Interim Force in Lebanon, the actual peace-keeping force.

Higgins, according to witnesses and U.N. officials here, was returning to the U.N. peace-keeping force's headquarters at Naqurah, 15 miles south of Tyre, when he was abducted at 2:15 p.m. local time (7:15 a.m. EST). He was driving alone in a U.N. car flying the blue-and-white U.N. flag, they said, following another U.N. vehicle, when three men armed with Kalashnikov assault rifles intercepted his car as the first vehicle rounded a curve near Ras Ain, four miles south of Tyre.

Witnesses said they saw the gunmen shove Higgins into the trunk of a red Mercedes and speed off with him along a dirt road forking off the main coastal highway.

When the three U.N. officers in the first vehicle realized Higgins was no longer following them, Goksel said, "they turned back and found Higgins' car empty."

The coastal stretch where the attack took place is contested by the Amal militia and the more radical, Iran-backed Hezbollah movement, which in 1986 launched rocket attacks against soldiers of the U.N. peace-keeping force.

Amal chief Nabih Berri went into a closed session with key military advisers this evening. Hassan Youssef, head of Amal's external relations department, said, "Amal will not leave a stone unturned in looking for and uncovering the perpetrators."

Amal has avoided attacks on U.N. troops in Lebanon, whom it sees as protectors against Israeli forces, which have established a security zone in southern Lebanon along the Israeli border.

"We will never acquiesce to their being subjected to any kind of danger," one Amal official said today. "This act only benefits Israel, although the actual tools carrying out the kidnaping could be non-Israeli."

A note delivered to a news agency in Beirut today raised new concerns about the two Scandinavian representatives of the U.N. Relief and Works Agency kidnaped south of Sidon on Feb. 5. The unsigned note accused the two, Jan Stening, 44, of Sweden, and William Joergensen, 57, of Norway, of being involved in activities of an intelligence service linked to a foreign country.

The statement said the two would be released only "when they are proven innocent."

Swedish and Norwegian foreign ministry spokesmen denied that either man was connected to intelligence activities.

The kidnaping of Joergensen and Stening prompted the U.N. agency to pull out some of its 14-member foreign staff from Beirut and temporarily move some of the operations to Damascus. The U.N. Development Program also postponed a planned assistance program for Lebanon pending the release of the two.

Many observers here were surprised to learn that any Americans were still in southern Lebanon. The widespread assumption had been that all official Americans had cleared out of Moslem-controlled areas after a wave of kidnapings swept Beirut in 1985.

Earlier today, Islamic Resistance fighters launched an attack against positions of the Israeli-backed South Lebanon Army near the Christian town of Jezzine, killing seven of them.