The Syrian army said yesterday it recaptured a gas field east of the central city of Homs that was seized by hardline Islamic State fighters earlier this month.

Syrian television showed footage of soldiers running and deploying in a vast desert area which it said was the Sha’ar gas field. The army said in a statement it retook the field after a “precise operation in which dozens of terrorists were killed.”

However, a source from the Islamic State said the fighters pulled out after destroying the field’s equipment and capturing at least 15 tanks and dozens of rockets which were used to guard the field.

“We pulled out because it was no longer good for us to stay. The goal was to get the tanks and rockets present at the field and we did,” he said. “There is no point in staying there and become an easy target for the regime and its warplanes.”

Islamic State, previously known as the Islamic State in Iraq and Levant, has advanced in Syria and taken over swathes of territory in neighbouring Iraq in what it has described as a bid to establish an Islamic caliphate.

Islamic State fighters decapitated most of the soldiers and hung heads outside base gates

At least 1,100 soldiers and pro-Assad fighters have been killed since the Islamic State intensified its attacks against government forces this month, the Syrian Observatory for Human Rights said.

During the assault on the field, the Islamic State fighters killed at least 270 soldiers, guards and staff and also killed at least 85 soldiers when they captured an army base in the province of Raqaa on Friday, the Observatory said. The fighters said they decapitated most of the soldiers and hung several heads outside the base gates.

Meanwhile, more and more Syrian rebels are defecting to join the ultra-hardline Islamic State insurgency, according to UN human rights investigators, in what they describe as a “Syrian-isation” of the al-Qaeda offshoot.

Members of the independent Commission of Inquiry on Syria, set up by the UN Human Rights Council in September 2011, informally briefed the United Nations Security Council on Friday ahead of submitting their latest report this week.

Brazilian chief investigator Paulo Pinheiro said the Islamic State – formerly known as the Islamic State in Iraq and the Levant, ISIL or ISIS – was undergoing a “Syrian-isation”.

“What began with a lot of foreign fighters, now you have authentic Syrian citizens,” Pinheiro told reporters. “We are seeing more Syrians coming to ISIS than before.”

He said there was an estimated 10,000 to 15,000 foreign fighters with various groups battling Syrian President Bashar al-Assad’s troops. Karen Koning AbuZayd, a member of the commission, said most of the Syrians joining Islamic State were defecting from other armed opposition groups fighting in the country’s civil war, now in its fourth year.

“They see it’s better, these guys are strong, these guys are winning battles, they were taking territory, they have money, they can train us,” AbuZayd told reporters.

After a successful offensive in Iraq, the Islamic State last month declared an “Islamic caliphate” in territory it controls in Iraq and Syria and vowed to expand.

Pinheiro said the group was a “good candidate” to be added to the inquiry’s confidential list of suspected war criminals in Syria. He said his message to the 15-member Security Council was: “Please do not forget Syria.”

UN investigators have not been able to enter Syria to carry out their work. They rely on interviews with refugees, telephone and Skype interviews with victims and witnesses in Syria, and photos, videos, satellite images, and forensic and medical reports. They seek corroboration to a level where they have “reasonable grounds to believe” an incident occurred.

“Once again we heard of the horrific scale of violations and abuses being carried out against innocent civilians,” British UN Ambassador Mark Lyall Grant said after the briefing, which Britain arranged. “All perpetrators should be held to account.”

The inquiry has called for the situation in Syria to be referred to the International Criminal Court. But an attempt by Western members of the council to do so was vetoed by Russia, a close ally of Syria, and China in May.

Without naming names, Pinheiro criticized “influential states” for their ambiguous position of supporting accountability for crimes in Syria and a push for a political solution, while at the same time helping the warring parties.

He said the full range of humanitarian breaches and gross rights violations were being committed by parties in Syria, including torture and summary executions.

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