The Islamic State of Iraq and Syria (ISIS) executed a woman by stoning her to death in the norther Syrian province of Raqa.

The Britain-based Observatory for Human Rights said Friday that the woman was accused of adultery.

The observatory further added that it was the first “execution” of its kind by the Islamic State of Iraq and Syria (ISIS) group in Syria.

Abu Ibrahim, an activist in Raqa province confirmed that the execution was carried out in a public square in the Tabaqa market area on Thursday evening.

“The situation is unbearable. Stoning is the worst punishment history has known. A quick death is more merciful,” another activist Hadi Salameh told AFP.

“The woman’s family did not know the sentence was going to be carried out at this time,” said Salameh.

ISIS first emerged in the Syrian conflict in late spring last year. The group has reportedly executed around 270 people after they seized a gas filed in Homs province on Thursday.