The United Nations High Commissioner for Human Rights Michelle Bachelet has called for the creation of humanitarian corridors in northwestern Syria, expressing horror at a Russian-backed regime offensive in the area.
Syrian troops are continuing an assault on the country's last major rebel enclave that has stoked fears of a humanitarian catastrophe as civilians flee.
Nearly 300 civilians have been killed in attacks this year in northwest Syria - 93% of them caused by strikes by the Syrian government and its ally Russia, according to Ms Bachelet.
"No shelter is now safe. And as the government offensive continues and people are forced into smaller and smaller pockets, I fear even more people will be killed," she said in a statement, adding that she is "horrified" by the unfolding humanitarian crisis.
"How can anyone justify carrying out such indiscriminate and inhumane attacks?" Ms Bachelet added.
The UN estimates that the offensive in the Idlib region has displaced about 900,000 people since the start of December, leaving huge numbers to sleep rough in the thick of winter.
The area, including parts of neighbouring Aleppo province, is home to some three million people, half of them already displaced from other parts of the country.
Ms Bachelet called on all parties to open humanitarian corridors for the safe passage of civilians, many of whom are stranded in rebel-held areas.
"They are now at a higher risk than ever, with little hope or guarantees for a safe and voluntary return to their areas of origin or areas of their choice," she said.
The UN said displacement camps were "overwhelmed by the sheer number of people seeking shelter" and many displaced "fearing for their lives have fled the camps".
Children have died of exposure in snow-covered camps and the UN has warned that the crisis could worsen if no ceasefire is reached to facilitate the relief effort.
"Over the past four days alone, some 43,000 newly displaced people have fled western Aleppo where fighting has been particularly fierce," UN spokesman David Swanson said.
Since the start of February, the displacement figure was a staggering 300,000, he added.
The wave of displacement is the biggest since the start of the civil war nearly nine years ago.
It is the largest exodus of civilians since World War II.
Syrian President Bashar al-Assad has said his forces will be relentless in their offensive to flush out the last rebel bastion.
"The battle for the liberation of the provinces of Aleppo and Idlib continue, regardless of all the hot air coming out of the north," Mr Assad said on state television, in reference to warnings by Turkey.
"We have won a victory over the fear they tried to instill in our hearts ... but we are fully aware that this liberation is not the end of the war," he said.
"But this liberation definitely does mean we have rubbed their faces in the dirt, as a prelude to their total defeat, which will come sooner or later," Mr Assad added.
Deadly incidents between regime forces and Turkey, which backs the rebels, have raised tensions near Syria's northern border, prompting Turkish President Recep Tayyip Erdogan to threaten Damascus.
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"The violence in northwest Syria is indiscriminate. Health facilities, schools, residential areas, mosques and markets have been hit," according to the UN head of humanitarian affairs and emergency relief, Mark Lowcock.
He said that basic infrastructure was falling apart, health facilities were being destroyed and the risk of disease outbreaks was high.
"The biggest humanitarian horror story of the 21st Century will only be avoided if Security Council members, and those with influence, overcome individual interests and put a collective stake in humanity first," Mr Lowcock said.
Russia, the main foreign broker in Syria, has vetoed countless resolutions on the conflict.
"The only option is a ceasefire," Mr Lowcock said.