Karam’s mother, Montera, put a box of ammunition in her youngest son’s car, restraining tears when he left at dawn for the front. It was reported that Islamic state militants stormed houses on the eastern edges of their native province of Svaida in southern Syria.

Monther joined two dozen other young men who had picked up arms and together they battled through to the edge of the nearby town of Rami. Fallen fighters lay strewn in the streets – the remains of the Isis militants in pieces after they detonated suicide vests.

A woman stumbled out of one of the the houses, repeating: “They slaughtered them.” Inside, she pointed to the bathroom.

“I felt in my heart that a crime had happened there,” Monther said. “I opened the door slowly, and I saw a mother holding her children, but it appeared she hadn’t been able to protect them from Daesh’s [Isis] gunshots.”

“I will never forget this scene all my life. No words can describe it. I knelt and wept in grief,” he said.

Residents of Sweida began burying their dead on Thursday, a day after the worst Isis atrocity in recent months claimed the lives of nearly 250 people – a toll that may yet rise.

Dozens were kidnapped and wounded, many are missing, and the slaughter has upended the peace of the ancestral homeland of the minority Druze sect, which had so far largely escaped the violence that devastated much of Syria.

Interviews with Sweida residents offered a glimpse into a brutal but well-planned, 12-hour attack that appeared to have evaded government security forces – and displayed the terror that Isis can still sow despite its repeated battlefield defeats and retreat across Syria and Iraq.

The raids began at about 4am on three simultaneous fronts. Under the cover of darkness, the militants infiltrated Druze towns and villages in the east and north-east of Sweida, some using local Bedouin as guides.

The militants knocked on doors – sometimes calling out locals by name – and slaughtered families. Meanwhile, they deployed snipers outside the town limits, and took up entrenched positions in the towns.

In most homes the militants left a single survivor as a witness to their brutality, said Monther. Some of the Isis fighters had tied their legs together in what appeared to be a symbolic statement that they would fight to the death without fleeing.

As news of the attack spread, local youth and militiamen took up arms. At the same time, at least four suicide bombers entered Sweida city. One bomber blew himself up at the vegetable market, another two in the city centre and the fourth detonated his vest in a building after being cornered by locals in the southern part of the city. About 30 people were killed by those attackers.

“In the beginning, the attack took us by surprise, but the heroic youth of Sweida rallied quickly in the centre of town and the villages that Daesh [Isis] had attacked,” said Osama Abu Dikar, a writer and journalist in the city. “These local fighters with basic capabilities fought real battles against Isis.”

He added: “The people of Sweida defended and fought with one heart. Today the city is recovering gradually. There is a very cautious calm, and there is an expectation of another attack.”

The coordinated assault raised questions over the competence of Syrian government forces, the capabilities of Isis and its ability to sow chaos – and the reconciliation deals that the regime of Bashar al-Assad has used to reclaim control of territory around the country.

Isis once controlled nearly half of Syria’s landmass at the zenith of its power in 2015. Since then it has lost vast tracts of territory to offensives by the US-led coalition and the Syrian military, retreating to desert hideouts and waging an insurgency.

The militants who attacked Sweida emerged from positions in the eastern Syrian desert to the east and north-east of the province. Scores of militants had arrived there in recent weeks – they had agreed to surrender the Yarmouk refugee camp, a sprawling suburb south of the capital Damascus, in exchange for safe passage to the area.

Their arrival to locations between 10 and 40 kilometres from the majority-Druze province led to great consternation among locals.

“Since then there have been civil society activists asking, why did you bring Isis east of Sweida?”, said one local journalist and activist.

Sweida is nominally under Syrian government control but has maintained a delicate neutrality that isolated it from the chaos raging throughout Syria.

The region has housed refugees fleeing other parts of the country, and while the government controls some of the entry points into the province, it has no checkpoints inside the provincial capital.

Security is loosely maintained by a hodgepodge of local militia, and tens of thousands of young Druze men have dodged their compulsory military service, a source of tension with the Assad regime.

But opposition activists argue that, with military victory within sight, government forces may eventually seek to re-establish total control of the region once more – arguing that the threat of extremists justifies their return, and allowing them to replenish the military’s ranks and clamp down on opponents.

Isis may have also sought to avenge an ongoing government offensive in southern Syria. The attacks occurred as forces loyal to Assad continued to wage a campaign in the neighbouring province of Deraa to reclaim control over all southern Syria.

Isis still controls a sliver of territory in Deraa in the Yarmouk river valley, near the border with the Israeli-occupied Golan Heights. The area has been the target of unrelenting airstrikes over the past few days to force the militants’ surrender.

But for the people of Suweida, who were attempting to provide security and survival for their families, not even the brutality of Isis would send them fleeing their ancestral homeland.

“We love peace and are tied to our land,” said Monther. “We will never leave here, or get on buses, or become displaced.

“Our roots are deep here, and we will stay, and we will be martyred here if they surround us on all sides.”