Seven hundred members of the al-Sheitaat tribe in Syria have been massacred by Islamic State forces, according to reports from the Syrian Human Rights Observatory.

Many of the executions were beheadings.

Reuters:

"Those who were executed are all al-Sheitaat," Observatory director Rami Abdelrahman said by telephone from Britain. "Some were arrested, judged and killed."

Reuters cannot independently verify reports from Syria due to security conditions and reporting restrictions.

Proclaiming a 'caliphate' straddling parts of Iraq and Syria, Islamic State has swept across northern Iraq in recent weeks, pushing back Kurdish regional forces and driving tens of thousands of Muslims, Christians and members of the Yazidi religious minority from their homes, prompting the first U.S. air strikes in Iraq since the withdrawal of American troops in 2011.

The insurgents are also tightening their grip in Syria, of which they now control roughly a third, mostly rural areas in the north and east.

An activist in Deir al-Zor who spoke on condition of anonymity told Reuters that 300 men were executed in one day in the town of Ghraneij, one of the three main towns of the al-Sheitaat tribal heartland, when Islamic State stormed the town earlier this week.

Another opposition activist from Deir al-Zor said residents of al-Sheitaat towns had been given three days to leave.

"Those who were executed during the storming of the al-Sheitaat area are around 300. The rest were killed in the course of the battles," he told Reuters on condition of anonymity to protect his identity.

Civilians fleeing al-Sheitaat towns had either taken sanctuary in other villages or travelled to Iraq, he said.

More than 170,000 people have been killed in Syria's civil war, which pits overwhelmingly Sunni Muslim rebels against President Bashar al-Assad, a member of the Shi'ite-derived Alawite minority, backed by Shi'ite militias from Iraq and Lebanon.

The Islamic State will never be defeated in Iraq until it is defeated in Syria. This has led to some bizarre suggestions.There are some real politik pundits who have suggested that we make an alliance with Syrian President Bashar Assad in order to defeat Islamic State.

That's not going to happen, but it points up how desperate the situation is becoming for the Syrian opposition.They've been fighting Islamic State as well as Hezb'allah and Assad's regular army and, have not been doing well.

Assad may end up fighting Islamic State anyway, but if things keep going the way they have been, the rebels will become more of a non-factor.