President Jose Napoleon Duarte, reacting to threats of more kidnapings against his family, sent three daughters and their children to the United States today aboard a U.S. Air Force jet.

The Salvadoran leader, whose daughter Ines was abducted by gunmen Sept. 10, said an unknown caller has telephoned several times since then claiming to have plans to kidnap more members of his family. As a result, they have been living "penned up" for security, he added. One bodyguard was killed and another wounded in the kidnaping of Ines.

"Even this morning there was a car following up on my daughter, and we had to take special precautions," Duarte said after seeing his children depart on the white-and-gray DC9 from the capital's Ilopongo military airport.

The president's decision to send threatened relatives to the United States was interpreted as a measure of concern here over the possibility of more attacks on Salvadoran or U.S. officials and their families by urban commandos working with the leftist rebels' Farabundo Marti National Liberation Front.

Although only a precaution, it amounted to indirect recognition that the threat is considered serious enough that already tight security measures surrounding Duarte and his family are seen as insufficient.

The rebel front, known by its Spanish initials as the FMLN, has not officially acknowledged kidnaping Duarte's daughter and has made no public threats against his other children. But Information and Culture Minister Julio Rey Prendes has said repeatedly that the rebels are holding the daughter and that the government is dealing with FMLN officials in two-way radio contacts designed to gain her freedom in exchange for captured rebel leaders.

U.S. soldiers also have been threatened with capture or assassination by the rebels' official Radio Venceremos, leading to what one diplomat described as sharply increased worry among U.S. personnel stationed here.

Four U.S. marines who were embassy guards and two other Americans were killed at a sidewalk cafe June 19 in assassinations later acknowledged by the FMLN. More recently, the front said in a communique that the main objective of its attack last Thursday on the Salvadoran Army's main training base was killing or capturing U.S. military advisers stationed there.

U.S. Ambassador Edwin Corr, who accompanied Duarte at the airport, said no American diplomats, military advisers or their families have left the country.

Duarte has four daughters, the kidnaped Ines Guadalupe Duarte de Duran, 35, and the three who left today with their children. He also has two sons. One of them, Alejandro, also accompanied the president as he saw off his daughters and grandchildren. "The men of the family and my wife are remaining here," he said.

El Salvador's ambassador in Washington, Pablo Alberge, said he was expecting Duarte's daughters, Maria Elena and Lorena, and Lorena's husband and child in Washington. He said the four would stay with him indefinitely at the embassy residence. One of Duarte's sons, Napoleon, a World Bank employe, already lives in Virginia.

Alberge also said Alejandro Duarte's wife and three children were flown to Miami and would remain there, while the other daughter of Duarte, Maria Eugenia, was flown to Atlanta with her husband and one child.

Corr said the plane was a scheduled government flight from U.S. Southern Command headquarters in Panama bound for Washington with possible stops on the way.

Duarte emphasized that he continues to act as president and that the government continues to function. This was seen as a response to reports that some military officers have expressed fears that the government, by negotiating with the kidnapers and seeming willing to bow to their demands for releasing prisoners, would appear weak and give the rebels the status of a legitimate belligerent in the war.

The president's decision came one year after his first meeting with rebel officials at the mountain town of La Palma for what was intended to be the beginning of a dialogue aimed at ending El Salvador's five-year-old civil war.

"The guerrillas have forgotten the spirit of La Palma," he declared.