The UN High Commissioner for Human Rights on Tuesday 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.
"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," Michelle Bachelet was quoted as saying in a statement.
The statement said Ms Bachelet was "horrified" by the unfolding humanitarian crisis.
"How can anyone justify carrying out such indiscriminate and inhumane attacks?" Ms Bachelet said.
The United Nations estimates that the offensive in the Idlib region has displaced about 900,000 people since the start of December.
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".
Syrian President Bashar Al Assad congratulated his forces Monday for recent gains in northwestern Syria that led to his troops consolidating control over Aleppo province, pledging to press ahead with a military campaign to achieve complete victory "sooner or later."
Mr Al Assad, who rarely appears in public, said in a televised address that the onetime economic hub of Aleppo, the provincial capital, will "return stronger than it was before."
"This liberation does not mean the end of the war, and does not mean the end of the schemes nor the end of terrorism or the surrender of enemies," Mr Al Assad said, seated behind an empty wooden desk and wearing glasses. "But it means that we rubbed their noses in the dirt as a prelude for complete victory and ahead of their defeat, sooner or later."
The address came amid an ongoing military advance in northwestern Syria that has sparked a humanitarian catastrophe which the UN's humanitarian chief Mark Lowcock warned "has reached a horrifying level." In a statement, he said the UN believes 900,000 people have been displaced since December 1, most of them women and children.
In the past few weeks, government troops backed by Russian air power have captured more than 1,500 square kilometres (580 square miles) in the northwest, consolidating their hold over Aleppo province after capturing over 30 villages and hamlets in the western countryside in a single day Sunday. The advance secured the provincial capital that had for years remained within range of opposition fire.
The new gains, along with securing a key highway through rebel territory, are set to better link northern and southern Syria, including the city of Aleppo, which was Syria's commercial center before the war. The highway, known as the M5, links the country's four largest cities and population centers and is key to controlling Syria.
The developments sparked late-night celebrations in the streets of Aleppo that continued through Monday, with state media showing residents waving flags and dancing in roads packed with vehicles.