Some 900,000 people have been forced from their homes and shelters in northwestern Syria since December, leaving huge numbers to sleep rough in the bitter cold.

The United Nations said half a million among them were children, some of whom have died of exposure in snow-covered camps.

It warned that the crisis could worsen unless a ceasefire is reached.

"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 told AFP on Tuesday.

A staggering 300,000 people have been displaced since the beginning of February, he said.

The wave of displacement is the biggest in nearly nine years of the civil war, which has forced half of Syria's population to flee their homes.

"The violence in northwest Syria is indiscriminate. Health facilities, schools, residential areas, mosques and markets have been hit," the UN head of humanitarian affairs and emergency relief, Mark Lowcock, said on Monday.

"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," Lowcock added.

More than 400 civilians have been killed since mid-December, according to the Syrian Observatory for Human Rights, with violence on Tuesday leaving at least another two dead.

Save The Children said seven children, including a baby only seven months old, have died in freezing temperatures and dire conditions in the camps.

"Those families who could take some of their belongings as they fled from their homes are reportedly burning whatever they could find, including pieces of furniture and whatever can be spared to stay warm for a short while," it said.