Firefighters who perished in the south tower of the World Trade Center on 11 September climbed almost 30 floors higher than previously believed, a tape of emergency radio transmissions has revealed.

Four people who have heard the recording said at least two firefighters reached the fiery crash site on the 78th floor.

The officers were helping the injured after United Airlines Flight 175 tore through the side of the skyscraper, The New York Times reported for Sunday editions.

The 78-minute tape was discovered in the rubble months ago but could not be played until fire officials and relatives signed a confidentiality agreement.

The agreement was requested because the tape may be used as evidence in the trial of Zacarias Moussaoui, who is accused of helping to plan the attacks.

Moussaoui, who faces the death penalty, admits to being an al-Qaeda member but denies involvement in the 11 September attacks.

Officials had thought that fire crews did not get beyond the 50th floor in either tower.

Families listen

The voices of at least 16 firefighters have been identified on the recording, and their families were invited to listen to the tape on Friday after signing agreements not to reveal content relevant to the Moussaoui trial.

Firemen helped the injured on higher floors

Debbie Palmer, whose husband, Battalion Chief Orio Palmer is heard on the tape, said listening to her husband's last moments had brought some relief.

"I didn't hear fear, I didn't hear panic," she told The New York Times.

"When the tape is made public to the world, people will hear that they all went about their jobs without fear, and selflessly."