ROME (AP) _ Rebel inmates holding 21 hostages in an island prison freed a 65-year-old ailing inmate today, while a mediator flew in to lead talks aimed at ending the siege, officials said.

The inmate, Claudio Rubini, was being treated in the fourth floor prison infirmary when six convicted murders armed with guns, knives and explosives took it over Tuesday on the island of Elba.

The officials said rebel prisoners decided to free Rubini apparently because he became hysterical and began screaming at them.

Three government prosecutors were reported to have continued telephone negotiations with the rebel inmates overnight, although no details were made available.

On Saturday, Justice Minister Giuliano Vassalli flew to Elba from Rome to take charge of the negotiations.

Also headed for the prison for possible mediation was Ernesto Olivero, 43- year-old director of a private social works institute looking after prisoners, the ANSA and AGI news agencies reported.

Olivero had been summoned from his Turin home by Vassalli after the families of the hostages had recommended him as a possible go-between, the news agencies said.

The reports said Olivero knows Mario Tuti, a convicted right-wing terrorist leading five other prisoners in the revolt.

The government on Friday refused to provide an escape helicopter demanded by the rebel inmates.

Prosecutor Antononio Costanza said in an interview with RAI that the prisoner's only demand was a helicopter and pilot to let them escape with two captives.

Authorities considered the demand, but ''there is no possibility the inmates can have a helicopter, at least the way things stand now,'' he said.

Negotiators insist that the inmates release Rossella Giazzi, a 29-year-old social worker and the only woman hostage.

Costanza said authorities would not use force or arms unless absolutely necessary.

''If those conditions come about, we're ready. But we must work to avoid a massacre,'' he said.

Prosecutors said earlier Friday that they were having a ''good dialogue'' by telephone with the captors.

Negotiators said 36 people remained in the infirmary: the six rebel prisoners, two inmates who said they had nothing to do with the revolt, seven infirmary patients and 21 prison staff members taken hostage, including the director.

Residents of the small island town carried placards signs in front of the city hall reading ''A helicopter for life'' and circulated petitions calling on the government to meet the demands of the prisoners.

The prison is located in Porto Azzurro, a resort village on the island in the Tyrrhenian Sea where Napoleon was exiled more than 170 years ago.