Terrifying first-person accounts of the final moments of the victims of the September 11 terror attacks have been heard in a US court.

Jurors deciding on whether Zacarias Moussaoui, who has confessed to being an al-Qaeda conspirator, should be given the death penalty have been played tapes of the seconds before lives were lost and heard testimony from relatives about the grief they have since suffered.

The jury heard a phone call made by a man caught high in the tower begging for help, and the grandfather of the youngest victim, a 2-year-old girl, has told how he was on the phone to her father as their plane slammed into the towers.

Kevin Cosgrove, who was working on the 99th floor of the south tower when it was hit, can be heard shouting, ‘’Oh my God…Aaaarrgggghhhh’’. His voice fades out amid crashing sounds caught on the phone before the line goes dead.

Earlier in the tape, Cosgrove, 46, vice-president of a company called Aon Corp is heard struggling for air as dark smoke filled his office.

"There are two of us in this office,’’ he says. ‘’We are not ready to die yet, but it is pretty bad. What the hell happened?’’

The jury at a US district court in Alexandria, Virginia, has already ruled that Moussaoui is eligible for the death penalty, finding that he contributed to the almost 3,000 deaths from the 9/11 attacks.

Now they are hearing testimony on the impact of the crimes as they decide whether he should be given the death penalty or a life sentence.

The grandfather of the youngest victim of the attacks described to the court how he was watching on television as the plane carrying his son and granddaughter hit the World Trade Center.

Lee Hanson, the grandfather of Christine Lee Hanson, said that his son Peter called him on September 11 and told him the plane he, his wife, Soo-Kim, and daughter were using to travel to Disneyland had been hijacked.

‘’I think they are going to try to crash this plane into a building," his father, 73, quoted him as saying. "He said: ‘Don’t worry Dad. If it happens, it will be quick.

‘’As we were talking, all of a sudden he stopped and said very softly: ‘Oh my God, oh my God, oh my God’.’’

"I looked over at the television set and saw a plane fly into the building," Mr Hanson said, describing how the hijacked jet rammed into the World Trade Center in New York.

Medical examiners asked him in the days after the attacks to retrieve DNA samples from the family home to identify remains.

It was, he said, "probably one of the worst things I ever did in my life. I was picking hair out of hair brushes, putting toothbrushes into bags".

The only remains that were ever found was a bone of his son, a few inches long.

The evidence came on the same day the judge in the death-penalty trial warned prosecutors against relying too heavily on emotional testimony to influence the jury.

US District Judge Leonie Brinkema cautioned prosecutors after complaints from the defence lawyers that a stream of victim-impact testimony last week would be overly prejudicial.

Judge Brinkema acknowledged that there was no way to avoid emotional testimony in this case, but reminded them that overly prejudicial testimony can be grounds for overturning a death sentence on appeal.

"You may pay a price for that down the road," she told prosecutors.

Prosecutors have said that about 45 such witnesses will testify, chosen from hundreds who wished to testify about the September 11 carnage.

The nine men and three women of the jury, have now sat through two harrowing days of witness testimony. They will reassemble today.

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