A month after his kidnappers said they would kill him, the fate of Marine Lt. Col. William R. Higgins, who turns 44 on Sunday, remains a mystery.

The last word on the American, kidnapped in south Lebanon 11 months ago, came Dec. 12 when his pro-Iranian Shiite Moslem captors said they had sentenced him to death for spying for Israel.No body has been found.

The Organization for the Oppressed on Earth, which claimed responsibility for Higgins' abduction, has also claimed responsibility for kidnapping 12 Lebanese Jews since 1984 and has said it killed eight of them. Four bodies have been found.

However, a Shiite source, familiar with the thinking of pro-Iranian factions, said he did not believe Higgins has been slain.

"He's too valuable a card to be wasted," said the source, speaking on condition of anonymity.

"He's an American officer who had held sensitive posts in the United States. He also served with the United Nations. This makes him an American national security interest and an international interest," the source said.

Higgins is one of 15 foreigners, including nine Americans, held hostage in Lebanon. The hostage held longest is American journalist Terry Anderson, chief Middle East correspondent for The Associated Press, who was kidnapped March 16, 1985.

Higgins, of Danville, Ky., led a 76-man U.N. military observer group monitoring the Lebanon-Israel border when he was kidnapped Feb. 17, 1988.

Two days later, the Organization of the Oppressed on Earth issued a statement claiming the abduction and charging that Higgins spied for the "satanic CIA."

The United States and the U.N. Interim Force in Lebanon, to which Higgins' observer group was attached, denied he was a spy.