The Syrian Observatory for Human Rights (SOHR) said on Tuesday that US-led air strikes in Syria have killed at least 1,171 people, mostly jihadists, since September 23.

The Daily Mail reports that 1,046 of the dead were members of the Islamic State (IS), which is the main target of the air campaign, and 72 were members of Al-Qaeda's branch in Syria, the Al-Nusra Front.

The group also said that 52 civilians, including women and children, were among those who had been killed in the air strikes.

The IS, which has seized large swathes of Iraq and Syria this year and declared a "caliphate" in the areas now under their control, have been accused of a wide range of atrocities, including mass executions, beheadings, and rape. The organisation's "annual report" released on March 31 this year contained disturbing chapter headings like assassinations, armed attacks, bombings, and prisoners freed.

The Christian Science Monitor stated in an article, "Even al-Qaeda has condemned the group's sweeping violence and disassociated themselves from it. However, its brand of mayhem, and the chaos of Syria, has attracted jihadist volunteers from across the region and Europe."

According to a recent UN report, which was based on around 500 interviews with witnesses, over 26,000 civilians have been killed or injured this year in the conflict in Iraq. It also stated that the militant group had performed "acts of violence of an increasingly sectarian nature" against Christians, Yazidis, Shiite Muslims, and other minority groups.

Paulo Barrozo, an expert on international criminal law, said that while the Taliban's brutality is limited to a single country, the IS "has torn up international borders in its attempt to create an Islamic caliphate."

He said, "They add a dimension of chaos and brutality to a region that's already volatile and that's in the best interest of no one. The population there is hostage to their brutality."

UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein agrees.

He said, "The array of violations and abuses perpetrated by [Islamic State] and associated armed groups is staggering, and many of their acts may amount to war crimes or crimes against humanity."

Lt. Gen. James Terry, the US commander in charge of the air strike mission, said last week that it would "at least take a minimum of three years" to defeat IS.