Out in the open in freezing weather, Mustafa Hamadi and his family settled into their makeshift tent in the village of Killi, in the northwestern Syrian province of Idlib - their second displacement in less than a year. The sub-zero temperatures on the night of February 11 kept them awake so, just before midnight, Mustafa moved the gas heater inside the tent.

By morning, Mustafa, his wife Amoun, their 12-year-old daughter Huda and their granddaughter Hoor, who was just three years old, were all found dead of carbon monoxide poisoning.

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According to Nizar Hamadi, Mustafa's brother who was texting with him that night, the tent - constructed of metal pipes and nylon sheets - had no proper ventilation and did little to insulate Mustafa's family from the cold.

"It must have been minus nine degrees Celsius (15.8 Fahrenheit) that night," Nizar told Al Jazeera. "My brother knew better than to bring a gas heater into an enclosed space with no air vents, but what choice did he have?"

Mustafa Hamadi and his daughter Huda pose in the snow a day before they were killed by carbon monoxide poisoning in Killi, Idlib province [Courtesy of Nizar Hamadi/Al Jazeera]

The Hamadi family, originally from Kafrouma village in the Maarat al-Numan countryside, were forced to leave their homes last summer and move further north amid an intensifying Syrian government aerial bombardment on Idlib, the last major opposition stronghold in the country.

Mustafa and Nizar settled in an empty unfinished school in the town of Binnish, some 8km (five miles) east of Idlib city, before Mustafa moved to Killi as the shelling escalated.

"The school is not fit for living," Nizar said. "But there is not a single house that has not been occupied by previous rounds of displaced people. Some rooms have three of four families living in them. The people displaced are like a snowball on the move, getting bigger every day."

Families sleeping without shelter

Backed by Russian air power, President Bashar al-Assad's troops in April last year launched a major offensive in Idlib, home to more than a million people, the majority of whom were transferred there en masse from other areas captured by government forces. The military push disrupted a fragile cooperation between Turkey and Russia - backing opposite sides in Syria's conflict - that had designated Idlib as a de-escalation zone.

The campaign continued as several ceasefires failed to hold up. In December, the Syrian government intensified its assault on the region in a bid to seize the strategic M5 highway, which runs through Aleppo and Idlib provinces and was once a major commercial route.

The offensive, which has also forced residents of western Aleppo to flee to Idlib, has killed hundreds of civilians and caused the largest single displacement of people since the war began in 2011, with at least 900,000 people forced to flee since December, according to the United Nations.

In addition to the indiscriminate bombing of civilians, the lack of adequate shelter has resulted in 82,000 people living in the freezing open air, settling under trees or in snowy fields, the UN said.

According to figures from the UN humanitarian body OCHA, 36 percent of newly displaced families are housed by relatives or find rental accommodation, while 17 percent found refuge in already overcrowded camps. At least 15 percent sought shelter in unfinished buildings and 12 percent are still "looking for shelter".

Nizar Hamadi, who is still living in the unfinished school in Binnish, said the reality for many people in IDP camps is "basically living under the trees in the summer, and setting up blankets and nylon sheets in the winter".

"Despite the fate that my brother and his family members faced, there has not been a single humanitarian organisation that responded to this tragedy by giving us provisions or tents," he said. "It's been like this for almost two months now. We need help but the sympathy seems to be reserved only for news headlines."

Babies freezing to death

Women and children - who comprise more than 80 percent of the newly displaced people - are again among those who suffer most.

Describing the situation in Syria as having reached a "horrifying new level", Mark Lowcock, the UN head of humanitarian affairs and emergency relief, said in a statement on Monday that the displaced are "traumatised" and "forced to sleep outside in freezing temperatures" because aid camps are full.

"Mothers burn plastic to keep children warm. Babies and small children are dying because of the cold."

In Kalbeet camp a few days ago, a five-month-old baby, Areej Majid al-Hmeidi, froze to death, according to Abu Anwar, an official and resident of the facility near the Syrian-Turkish border.

Speaking to Al Jazeera, Anwar said Areej's family do not want to talk to media because "they blame themselves for not keeping her warm enough to stay alive".

The conditions here are "insufferable", he said.

"People are burning rubbish to keep themselves warm," Anwar added. "There are 800 families here, or around 5,500 people, and there is only one organisation that is helping us by supplying us with water."

'Absolute silence, lack of action'

Sara Kayyali, a Human Rights Watch researcher on Syria, said the country's northwest is facing an "unprecedented humanitarian crisis".

One issue, she told Al Jazeera, is the "scale of displacement [that] is simply beyond what the humanitarians are capable of responding to".

"The other issue is that the violence - shelling and in some cases air strikes - are not just resulting in the massive displacement, but also impacting the ability to provide shelter and food in a sustained manner," she continued.

Mayada Qabalan, a mental health worker with the Union of Medical Care and Relief Organizations (UOSSM) who works at a hospital in Idlib's Sarmada, said the conditions for displaced people have reached "a breaking point".

"What I've seen with my own eyes is heartbreaking," she told Al Jazeera. "Families are sleeping under trees with no cover. Just a few days ago we found a family displaced from Taftanaz, some 17km (11 miles) northwest of Idlib, who were living out in the cold."

A view of the makeshift tents and houses at a refugee camp in Idlib [Anadolu Agency]

Tents cost $150 each, Qabalan said, but humanitarian groups are severely lacking in resources and manpower to offer help.

"Aid organisations don't have the capacity for providing for these newly displaced people and the disastrous situation they face," she said.

Kayyali said while the stories coming out of Idlib and western Aleppo are not new in the Syrian conflict, they are "surprising in the absolute silence and lack of action that follows".

"It is as though people are watching and waiting when they could be acting to save millions of civilians that are effectively trapped," she said.