Antakya, Turkey - As tens of thousands of Syrian civilians continue to flee their homes in northwest Idlib for the Turkish border, humanitarian and aid organisations are warning of an increasingly dire situation.

More than 950,000 Syrians have been forced from their homes since December, according to the United Nations, in the wake of an intensified military operation by Syrian government forces and their allies to retake the last rebel stronghold in the country.

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The vast majority of the displaced - at least 81 percent - are women and children.

Since December, more than 200 refugee camps have been set up in Idlib, along the Turkish border, to accommodate some of the displaced civilians, known as internally displaced people (IDP), according to aid workers. But the camps lack basic sanitation and are far over their capacity, forcing tens of thousands to sleep outside in sub-zero temperatures, resulting in a number of children freezing to death.

Mark Lowcock, head of the UN Office for the Coordination of Humanitarian Affairs (OCHA), has described the situation in Idlib as the "biggest humanitarian horror story of the 21st century" and called for an immediate ceasefire.

Tents housing internally displaced people in Atma camp in Idlib governorate [Umit Bektas/Reuters]

Syrian forces, backed by Russian airpower, have rapidly advanced into Idlib and have retaken the strategic M5 highway since December, in violation of agreements signed in 2017 and 2018 between Turkey and Russia, which had designated the province as a "de-escalation" zone.

The operation first began in April last year but stuttered in August amid attempts to broker ceasefires.

Maarouf Semua, an aid worker at Turkey's IHH Humanitarian Relief Foundation, said a total of 1.2 million Syrians have been displaced from their homes since April last year, at least half of them for the second and third time in Syria's nine-year war.

"In the last six months, the humanitarian situation has intensified beyond imagination," he said.

"We and other aid organisations are working round the clock to provide assistance to the IDPs but we simply do not have the capacity to cater to such a large number - it's an impossible task," he said.

The biggest priority, according to aid workers, is providing adequate housing to refugees as temperatures plunge below zero.

IHH is building two-room houses made of cement blocks, measuring 25 square metres (269 square feet). Of the planned 10,000 structures, 650 have been built so far, Semua said.

Those who get such housing are among the lucky ones, as most camps face a "dire lack" of even simpler shelters such as tents, according to Kutaiba Sayed Issa, director of Syrian aid group Violet.

"Whoever has one for just his family is considered to have a castle. I was in Idlib last week and saw with my own eyes 50 people living in one room, and four families to a tent," he told Al Jazeera from his office in Antakya, Turkey.

Three aid workers from Violet organisation killed three months ago [Linah Alsaafin/Al Jazeera]

Even when there were enough tents, the lack of infrastructure for basic needs such as water, sanitation and electricity remained a challenge, said Issa.

"Out of the 200 or so new camps set up, not one of them has a working toilet," said Issa, who was in Idlib last week. "Women are forced to wait until the middle of the night and go in groups just to relieve themselves out in the open."

"I have seen many cases of nervous breakdowns, mostly afflicting women and children. They go from having a roof over their heads to living out in the open or in shoddy tents without any of their personal belongings, jobs and homes, begging for a bite to eat. Tens of thousands of children are suffering from panic attacks and bedwetting."

Issa said he was shocked at the lack of international attention to the situation in Idlib, which is home to almost four million people and under the control of armed opposition groups, including Hay'at Tahrir al-Sham (HTS), a former al-Qaeda affiliate. He said the lives of hundreds of thousands of civilians must not be put at risk for the sake of defeating a few thousand HTS fighters.

"Back when Aleppo was under siege in 2016 there was more worldwide awareness and attention, but regarding the situation in Idlib there has been little to no reaction," Issa said.

IHH supplies mattresses, food baskets and canvases for internally displaced Syrians in Idlib [Linah Alsaafin/Al Jazeera]

Aid workers said the main challenges they face in Idlib was the constant aerial bombardment by Syrian government forces and Russian warplanes, as well as a lack of resources.

"You've got a very fluid situation on the ground so it is incredibly difficult and complex environment to operate any kind of humanitarian response efforts," said David Swanson, spokesman for OCHA, calling for more resources to address the crisis.

Of the $3.3bn aid the UN had requested from the international community in 2019, only 65-68 percent was received, leaving a huge funding gap. For the current crisis, Swanson said the UN has put out a request for the international community to raise more than $500m to address the humanitarian needs of 1.2 million people over a six-month period.

A third challenge, Swanson said, is that aid organisations have to rely on Syrian aid workers to provide assistance to those in Idlib.

The UN has no formal presence on the ground in northwest Syria and the OCHA works with more than 15,000 Syrian aid workers - whom Swanson described as the "unsung heroes" - to assist vulnerable people.

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But these workers are not immune to the unfolding reality in Idlib, and many of them have had their own families displaced, which Swanson said undermines the UN's ability to respond as effectively as it would like to.

Some Syrian aid workers have also been killed in the fighting.

Violet's Issa said six of his 2,000-member strong organisation have been killed over the past year.

"Three of them - young men under the age of 22 - were medics and were killed three months ago in Maaret al-Numan after being targeted by the Syrian regime," he said.

"The most recent death we suffered was four weeks ago, when our member Zahwan Tamma was killed in shelling in the hospital in the city of Ariha, before it was overtaken by Assad forces."

"In Idlib, life is a constant state of horror," Issa said. "I could not sleep during my last visit there, as there was nonstop shelling. I don't have the words any more to describe how bad the situation is."