Egyptian authorities have executed nine men convicted over the 2015 killing of the country's chief prosecutor, a prison source and a lawyer said on Wednesday.

The men were among a group of 28 who were sentenced to death in the case in 2017. Public prosecutor Hisham Barakat was killed in a car bomb attack on his convoy in the capital, Cairo.

Egypt blamed the Muslim Brotherhood and Gaza-based Hamas militants for the operation. Both groups denied having a role.

Rights group Amnesty International had appealed on Tuesday for authorities to halt the executions, citing testimony by the defendants that they had been secretly detained and tortured into confessing.

Since 2013, the year that then-army chief Abdel Fattah al-Sisi military ousted President Mohamed Mursi of the Muslim Brotherhood, Egyptian criminal courts have issued hundreds of death sentences.

Only a small proportion have been carried out, though the rate of executions has risen since 2015, rights activists say.

El Watan newspaper said the executions were carried out on Wednesday morning.

They followed six others carried out earlier this month. Three of those were convicted over the killing of a police office in September 2013, and three over the killing of a judge's son the following year.

In both cases the defendants or their lawyers had said torture was used to extract confessions, according to Amnesty International.

Sisi, who was elected president in 2014 and re-elected last year, says he is working to bring stability and security to Egypt following the turmoil of the 2011 uprising.