When the black-cowled gunmen of the 'Islamic State' infiltrated the suburbs of Palmyra on 20 May, half of Assad Sulieman’s oil and gas processing plant crews – 50 men in all - were manning their 12-hour shift at the Hayan oil field 28 miles away. They were the lucky ones. Their 50 off-duty colleagues were sleeping at their homes next to the ancient Roman city. Twenty-five of them would soon be dead, among up to 400 civilians – including women and children – who would die in the coming hours at the hands of the Islamist militia which every Syrian now calls by its self-styled acronym ‘Daesh’.

Oil engineer ‘Ahmed’ – he chose this name to protect his family in Palmyra – was, by chance, completing a course at Damascus University on the fatal day when Palmyra fell. “I was appalled,” he said. “I tried calling my family. It was still possible to get through on the phone. They said ‘Daesh’ (also known as Isis) wasn’t allowing anyone to leave their home. My brother later went onto the street. He took pictures of bodies. They had been decapitated, all men.

Destruction of the Jezaa gas and oil processing plant (Nelofer Pazira)

“He managed to send the photographs out to me from [the Isis-controlled city of] Raqqa on the internet which is the only communications working there.”

Some of the photographs are too terrible to publish. They show heads lying several feet from torsos, blood running in streams across a city street. In one, a body lies on a roadway while two men cycle past on a bicycle. So soon after the capture of Palmyra were the men slaughtered that shop-fronts can still be seen in the photographs, painted in the two stars and colours of the red-white-and-black flag of the Syrian government.

“The Daesh forced the people to leave the bodies in the streets for three days,” Ahmed continued. “They were not allowed to pick up the bodies or bury them without permission. The corpses were all over the city. My family said the Daesh came to our house, two foreign men – one appeared to be an Afghan, the other from Tunisia or Morocco because he had a very heavy accent – and then they left. They killed three female nurses. One was killed in her home, another in her uncle’s house, a third on the street. Perhaps it was because they helped the army [as nurses]. Some said they were beheaded but my brother said they were shot in the head.”

Ancient monuments under Isis threat Show all 8 1 /8 Ancient monuments under Isis threat Ancient monuments under Isis threat Isis seizes Palmyra The ancient oasis city of Palmyra Getty Ancient monuments under Isis threat Isis seizes Palmyra A partial view of the ancient ruins Getty Ancient monuments under Isis threat Isis seizes Palmyra The ancient Palmyra theater Reuters Ancient monuments under Isis threat Isis seizes Palmyra A view of the ancient Syrian city of Palmyra Getty Ancient monuments under Isis threat Isis seizes Palmyra The Temple of Bel Reuters Ancient monuments under Isis threat Isis seizes Palmyra Palmyra's famous graves AP Ancient monuments under Isis threat Isis seizes Palmyra The ancient castle Getty Ancient monuments under Isis threat Isis seizes Palmyra A sculpture depicting a rich family from the ancient Syrian oasis city of Palmyra, displayed at the city's museum Getty

In the panic to flee Palmyra, others perished when their cars drove over explosives planted on the roads by the Islamist gunmen. One was a retired Syrian general from the al-Daas family whose 40-year old pharmacist wife and 12-year old son were killed with him when their car’s wheels touched the explosives. Later reports spoke of executions in the old Roman theatre amid the Palmyra ruins.

The director of the Hayan gas and oil processing plant, Assad Sulieman, shook his head in near-disbelief as he recounted how word reached him of the execution of his off-duty staff. Some were, he believes, imprisoned in the gas fields which had fallen into the hands of the ‘Islamic State’. Others were merely taken from their homes and murdered because they were government employees. For months prior to the fall of Palmyra, he had received a series of terrifying phone calls from the Islamists, one of them when gunmen were besieging a neighbouring gas plant.

He said: “They came on my own phone, here in my office, and said: ‘We are coming for you.’ I said to them: ‘I will be waiting’. The army drove them off but my staff also received these phone calls here and they were very frightened. The army protected three of our fields then and drove them off.” Since the fall of Palmyra, the threatening phone calls have continued, even though 'Daesh' have cut all mobile and landlines in their newly-occupied city.

Director of the Hayan gas and oil processing plant, Assad Sulieman, shows the damage to oil fields (Nelofer Pazira)

Another young engineer at Hayan was in Palmyra when the 'Islamic State' arrived. So fearful was he when he spoke that he even refused to volunteer a name for himself. “I had gone back to Palmyra two days before and everything seemed alright,” he said. “When my family told me they had arrived, I stayed at home and so did my mother and brother and sisters and we did not go out. Everyone knew that when these men come, things are not good. The electricity stopped for two days and then the gunmen restored it. We had plenty of food – we were a well-off family. We stayed there a week, we had to sort out our affairs and they never searched our home.”

The man’s evidence proved the almost haphazard nature of Isis rule. A week after the occupation, the family made its way out of the house – the women in full Islamic covering – and caught a bus to the occupied city of Raqqa and from there to Damascus. “They looked at my ID but didn’t ask my job,” the man said. “The bus trip was normal. No-one stopped us leaving.” Like Ahmed, the young oil worker was a Sunni Muslim – the same religion as ‘Daesh’s’ followers – but he had no doubts about the nature of Palmyra’s occupiers. “When they arrive anywhere”, he said, “there is no more life”.

The ancient Palmyra theater (Reuters)

Syria’s own oil and gas lifeline now stretches across a hundred miles of desert from Homs in the midlands to the strategic oil fields across the broiling desert outside Palmyra. It took two hours to reach a point 28 miles from Palmyra; the last Syrian troops are stationed eight miles closer to the city.

To the west lies the great Syrian air base of Tiyas – codenamed ‘T-4’ after the old fourth pumping station of the Iraqi-Palestine oil pipeline – where I saw grey-painted twin-tailed Mig fighter bombers taking off into the dusk and settling back onto the runways. A canopy of radar dishes and concrete bunkers protect the base and Syrian troops can be seen inside a series of earthen fortresses on each side of the main road to Palmyra, defending their redoubts with heavy machine guns, long-range artillery and missiles.

Syrian troops patrol the highway every few minutes on pick-up trucks – and make no secret of their precautions. They pointed out the site of an improvised explosive device found a few hours earlier - more than 30 miles west of Palmyra. Further down the road was the wreckage of truck bombs which had been hit by Syrian rocket-fire. Assad Sulieman, the gas plant director, declares that his father named him after President Bashar al-Asasad’s father Hafez. He described how Islamist rebels had totally destroyed one gas plant close to Hayan last year, and how his crews had totally restored it to production within months by using cannibalized equipment from other facilities. His plant’s production capacity has been restored to three million cubic metres of gas per day for the country’s power stations and six thousand barrels of oil for the Homs refinery.

Ancient sites in Palmyra (EPA)

But the man who understands military risks is General Fouad – like everyone else in the area of Palmyra, he prefers to use only his first name – a professional officer whose greatest victory over the rebels on a nearby mountain range came at the moment his soldier-son was killed in battle in Homs. He makes no secret of “the big shock” he felt when Palmyra fell. He thinks that the soldiers had been fighting for a long time in defence of the city and did not expect the mass attack. Other military men – not the general – say that the ‘Islamic State’ advanced on a 50-mile front, overwhelming the army at the time.

“They will get no further,” General Fouad said. “We fought them off when they attacked three fields last year. Our soldiers stormed some of their local headquarters on the Shaer mountain. We found documents about our production facilities, we found religious books of Takfiri ideas. And we found lingerie.”

What on earth, I asked, would the Islamic State be doing with lingerie? The general was not smiling. “We think that maybe they kept captured Yazidi women with them, the ones who were kidnapped in Iraq. When our soldiers reached their headquarters, we saw some of their senior men running away with some women.”