President Jose Napoleon Duarte today offered to release 22 rebel prisoners in exchange for his kidnaped daughter, her friend and two dozen abducted mayors.

"I have completed and obtained the support necessary to liberate them, and so I am prepared to set them free in exchange for Ines Guadalupe Duarte, Ana Cecilia Villeda and the mayors," said a voice identified by experienced Salvadoran observers as that of Duarte speaking on a short-wave radio.

"I need proof of the state of health of Ines Guadalupe Duarte and Ana Cecilia Villeda," he added during the monitored transmission. In what appeared to be a response from the rebel captors, another voice said, "We will give proof."

The offer from Duarte's government marked the first official confirmation that the president is getting ready to release a large number of guerrilla prisoners to obtain the release of his daughter. Government and military sources previously had expressed belief that he would accede to the kidnapers' demands and said the military command approved despite reticence from some officers concerned about setting a precedent.

The radio broadcast's mention of "necessary support" was interpreted as a reference to the high command's backing.

Ines Guadalupe Duarte Duran, 35, and her 23-year-old friend, Ana Cecilia Villeda, were kidnaped by gunmen in San Salvador on Sept. 10. Their abductors later identified themselves in radio calls to the government as the Pedro Pablo Castillo Front of the Farabundo Marti National Liberation Front, the guerrilla alliance in El Salvador's civil war.

According to government sources and a previous monitored radio conversation, the captors have demanded the release of 34 rebel prisoners in return for the president's daughter and her friend. The government also has sought to include two dozen abducted mayors in the exchange, but it is unclear whether the kidnapers have agreed.

There was no immediate government comment on the monitored conversation.

The rebels have demanded that all those on the list of 34 who are not turned over be accounted for. The speaker said to be Duarte gave detailed explanations for a number of these, reporting, for example, that several were killed in clashes with the Army as long ago as 1982. But he acknowledged that some could not be traced.

"For all those people whose whereabouts is unknown, we have exhausted our means and elements of investigation," he said. The rebels' official Radio Venceremos frequently has accused the government of killing guerrilla prisoners. Failure to account for some of the 34 could become a stumbling block for the release of Duarte's daughter.

Three prisoners were released Tuesday, but it was unclear whether they were among the 34. Government sources had said earlier that four prisoners were released Tuesday.