A cleric read the verdict before the truck came and dumped a large pile of stones near the municipal garden. Jihadi fighters brought in the woman, clad head to toe in black, and half buried her in the ground.

Then they told residents who had gathered around the scene to carry out the sentence: Stoning to death for the alleged adulteress.

None in the crowd stepped forward, said a witness to the event in a northern Syrian city. So the jihadi fighters, mostly foreign extremists, did it themselves, pelting Faddah Ahmad with rocks.

Death by stoning: A cleric uses a loudspeaker to read out the sentence handed to Faddah Ahmad

Public execution: Hundreds of locals were gathered at the scene of the killing, which took place at 11pm

Carrying out the sentence: A witness said none in the crowd stepped forward, and it was left to the mainly foreign-born fighters from the Islamic State to pick up the stones and bludgeon Ms Ahmad to death

'Even when she was hit with stones she did not scream or move,' said an opposition activist who said he witnessed the stoning near the football stadium in Raqqa, the main Syrian stronghold of the Islamic State group.

The July 18 stoning was the second in a span of 24 hours. A day earlier, 26-year-old Shamseh Abdullah was killed in a similar way in the nearby town of Tabqa by IS fighters.

She was also accused of having sex outside marriage, the Associated Press reports.

The killings are believed to be the first of their kind in rebel-held northern Syria, where IS jihadis have seized large swaths of territory, terrorising residents with a strict interpretation of Islamic law, including beheadings and cutting off the hands of thieves.

The jihadis recently tied a 14-year-old boy to a cross-like structure and left him for several hours in the scorching summer sun before bringing him down - punishment for not fasting during the Muslim holy month of Ramadan.

This pictured posted by the Raqqa Media Centre, a Syrian opposition group, shows fighters from the Islamic State on parade in the city of Raqqa. Activists have reported two cases of stoning this month in that province

The stonings in Syria last month were not widely publicised at the time, but in the following days three photographs appeared online which appeared to document the grisly spectacle.

The circumstances of the photos were consistent with other AP reporting.

Pictures posted on a newly-created Twitter account showed dozens of people gathered in a square, a cleric reading a verdict through a loudspeaker and several bearded men with automatic rifles either carrying or collecting stones.

'Even when she was hit with stones she did not scream'

'A married woman being stoned in the presence of some believers,' read the caption of the photographs on the Twitter account, which has since been suspended.

Abu Ibrahim Raqqawi, the activist who witnessed Ahmad's stoning, told AP that locals were angry to see foreign fighters impose their will on the community.

'People were shocked and couldn't understand what was going on. Many were disturbed by the idea that Saudis and Tunisians were issuing (such) orders,' he said in an interview via Skype.

Ms Ahmad, he said, appeared unconscious, and he had overheard that she was earlier taken to a hospital where she was given anaesthesia.

The stoning took place after dark, he said, at about 11pm. He could not see blood on the body because of the black clothes she was wearing.

Ms Ahmad did not scream or shake. She died silently. 'They then took the dead body in one of their cars and left,' he said.

The first case of stoning occurred in the town of Tabqa. A day after the July 17, 2014, stoning of Shamseh Mohammed Abdullah, 26, Faddah al-Sayed Ahmad was stoned to death in the provincial capital of Raqqa

The two cases were first reported by the Britain-based Syrian Observatory for Human Rights, which collects information through a network of activists around the country. Bassam Al-Ahmad, a spokesman for the Violations Documentation Centre, a Syrian group that tracks human rights violations, also confirmed the stoning.

An activist based in the northern province of Idlib, who collects information from other activists in northern Syria, said Ms Ahmad was a widow.

A man who asked to be identified as Asad, a pseudonym, for fear of repercussions, said that in the other stoning, in Tabqa, residents also refused to take part, and that the act was carried out by Islamic State members.

The U.S. Embassy in Syria, in a statement posted on its Twitter account, condemned the 'barbaric stoning' of a woman in Tabqa.

International human rights groups did not report the stoning, and Human Rights Watch said it had no independent confirmation.

'It is a very worrying trend if true,' said Human Rights Watch researcher Lama Fakih.

The Islamic State group has 'imposed incredibly restrictive rules on the civilian population which have served to make women and girls particularly vulnerable and to quite clearly discriminate against them,' she said, adding that the reports of the stoning were the first the group had received out of Syria.