In a stunning last-minute intervention, the US Supreme Court has delayed the execution of convicted killer Troy Davis in a US prison seconds before he was to receive a lethal injection.

As Davis supporters drummed and chanted and the victim's tension-racked family prayed outside a Georgia death-row jail, confusion reigned over whether the delay was yet another stay after a last-ditch legal appeal or merely a pause.

But the Supreme Court put the execution "on hold" to deliberates on another appeal for a stay. US journalists reported the Supreme Court would make another ruling on the execution at 8.30pm, local time, (10.30am AEST), but the time passed with no further news.

Davis was to die by lethal injection in the Jackson prison at 9am (AEST).

"Stay!", tweeted a New York Times reporter at 9.02am.

A cheer went among up Davis supporters, she reported, while family members of his alleged victim, policeman Mark MacPhail, huddled in prayer.

CNN reported Anneliese MacPhail, the mother of the slain policeman, had been told the US Supreme Court was reviewing a last-minute plea for stay.

If a stay is granted, it will be the fourth time Davis has been spared. Two stays came just hours before he was due to be executed. This, however, would be the most nerve-shattering, for both his backers and the McPhail family.

Supporters of clemency for Davis include the Pope, South African Archbishop Desmond Tutu, former US president Jimmy Carter and a former FBI director William Sessions.

Davis, 42, was convicted of the fatal shooting in 1989 of off-duty police officer Mark MacPhail in the car park of a Savannah takeaway.

Mr MacPhail, a 27-year-old father of two infants, had sought to break up an argument when he was shot in the chest and face.

Davis's arrest and subsequent conviction - despite the absence of a weapon, DNA evidence and any surveillance footage - signalled the beginning of his tortuous trek through the US judicial system, during which time his case was subjected to a series of appeals and at one stage reached the Supreme Court.

A parole board yesterday rejected a last-ditch plea to stay the lethal injection.

More to come