Fighters from Al-Qaeda-breakaway group the Islamic State have stoned a woman to death for adultery, in the first such execution of its kind in rebel-held northern Syria.

The stoning, first reported by the Syrian Observatory for Human Rights and confirmed to Al Jazeera by two activists, took place in a public square Thursday evening in the town of Tabaqa, Raqqa province. Activists said the woman was tried in an Islamic sharia court, but that witnesses to her alleged offense were never identified and that the man involved was not charged with any crime.

“Mystery surrounds the whole thing,” said Abu Khalil, an activist in Raqqa who runs the anti-Islamic State group Raqqa is Being Slaughtered Silently, which obtained a cell phone photograph that purports to depict the incident.

In a tweet, the U.S. Embassy in Syria – which no longer has a presence in the country – condemned the “barbaric” stoning “in the strongest possible terms."

While far from the first instance of Islamic State brutality in Raqqa, the stoning has exacerbated fears the hardline group will govern brutally and without accountability across Syria and Iraq, where it led a coalition of Sunni forces to takeover large swathes of the north and west in a lightning offensive last month.

Raqqa, where the Islamic State established its de facto capital last year, is widely considered a glimpse of what’s to come in other regions that have fallen under IS control.

Some war-weary Raqqans were initially receptive to Islamic State rule when the group first swept into the province, unaware of its reputation for brutality that even Al-Qaeda has criticized. But after working alongside secular and Islamist rebel factions to wrest control of Raqqa from the Syrian regime, the Islamic State fended off rival factions and began to impose its radical brand of Islamic law, setting off mass protests.

With no coercive force to back them up and other rebel factions spread very thin in their battle against the Syrian government, Raqqans are powerless to expel their extremist rulers.

The group’s leader, Abu Bakr al-Baghdadi, has already declared the region to be a restored 7th century-style caliphate that will ultimately stretch across the region.

The same day the Observatory reported the stoning, it also announced the Islamic State now controlled over 35 percent of Syria, following a string of victories. This week, Islamic State fighters took over parts of Deir e-Zour, the last major city needed to bridge its established holdings in Syria with its recent, still-fragile gains in Iraq.

Also Friday, the United Nations accused Islamic State fighters in Iraq of war crimes that include executing religious leaders, forcibly recruiting children and raping women.

"Every day we receive accounts of a terrible litany of human rights violations being committed in Iraq against ordinary Iraqi children, women and men, who have been deprived of their security, their livelihoods, their homes, education, healthcare and other basic services," said U.N. High Commissioner for Human Rights Navi Pillay.

Separately on Friday, Islamic State issued orders to Christians in the Iraqi city of Mosul, which it captured last month, to convert to Islam or pay a tax, or otherwise face death.