Aid group the Norwegian Refugee Council on Wednesday warned of the "worst catastrophe" in Syria's civil war if no ceasefire is reached for the embattled rebel bastion of Idlib.
Russia-backed regime forces have chipped away at the militant-ruled region since December, forcing almost 700,000 people to flee their homes toward the closed Turkish border.
"This is the largest displacement in the worst war of our generation," said NRC chief Jan Egeland.
Syria's war has already killed more than 380,000 people and displaced millions since the conflict started in 2011 with the brutal repression of anti-government protests.
Government loyalists have shown no sign of letting up in their latest offensive, on the rebel stronghold of some three million people, half of whom have already been displaced from elsewhere.
Egeland urged a ceasefire deal for the Idlib region to avert a disaster in what he described as "the world's largest refugee camp".
"Our fear now is that a full-on offensive could lead to the worst catastrophe of Syria's brutal war, an out-and-out bloodbath for displaced civilians," he said.
"Above all we need a ceasefire and talks."
- 'Desperate need'' -
Idlib has been a refuge in recent years for people fleeing or being evacuated from previously rebel-held areas in other parts of the country.
But now that Idlib is under attack, its residents have nowhere else to go.
"Turkey must be supported to offer safe passage to the women, men, and children fleeing the violence across its border and into areas it controls in northern Syria," Egeland said.
Numerous aid organizations have called for a halt to the violence in northwest Syria, where the newly displaced are sleeping in cars and in fields in often cold temperatures.
"The situation is so dire today that families in desperate need of food, blankets, and mattresses are being turned away from overcrowded shelters," he said.
"Rent prices have skyrocketed, and we hear reports of up to a dozen families sharing a single apartment," Egeland added.
The UN World Health Organization said Tuesday that the healthcare personnel were struggling to keep up.
"Increasing numbers of displaced persons are cramped in a small geographical area, putting an enormous strain on health responders," it said.
In the rest of the region, some 72 healthcare centers have been suspended since December 1.
"This means that more than 100,000 medical outpatient consultations are suspended, nearly 11,000 trauma patients are not served every month and 1,690 major surgeries will not be performed this month, endangering patient lives," it said.
The UN agency confirmed five separate attacks on healthcare services, which resulted in 10 deaths.
