Isis jihadists in Syria's northern Raqa province have stoned to death two women accused of adultery.

The first sentence was imposed by an Isis Islamic court on an unnamed woman reported to be in her thirties and carried out by a mob in the town square in the Al Tabaqa market area on Thursday.

"The Islamic State carried out its first sentence of death by stoning against a woman in Tabaqa, accusing her of adultery," the UK-based Syrian Observatory for Human Rights said.

On Saturday, Isis carried out the stoning of a second woman, believed to be 26 years old. In at least one of the cases, the stoning was reportedly carried out because the woman's new husband discovered that she was not a virgin.

"IS carried out the punishment of Al Rajem (stoning to death for adultery) against a woman from the city of Al Tabaqa, where she was brought after Eshaa Prayer to the popular market in the city of Al Tabaqa and stoned to death," the Observatory said.

"It was cleared that the woman was 26 years old and she was a widow named SH.M.A."

The executions are reported to be the first of their kind carried out by the militant Islamist group, which is also known as the Islamic State. The group has established a self-proclaimed Islamic "caliphate" straddling parts of Syria and Iraq.

Activist Hadi Salameh told AFP: "The situation is unbearable. Stoning is the worst punishment history has known. A quick death is more merciful. The woman's family did not know the sentence was going to be carried out at this time."

Salameh added that local residents are fearful of Isis but are afraid of the consequences of speaking out against its brutal tactics and human rights abuses.

Isis renewed its oppression of Iraq's ancient Christian community in Mosul this week, stating that they must pay tax, convert to Islam or leave in order to avoid "death by the sword".

In a statement read out at mosques in Mosul, Isis said: "We offer them three choices: Islam; the dhimma contract - involving payment of jizya; if they refuse this, they will have nothing but the sword."