Eight humanitarian aid organizations on Wednesday called for an immediate ceasefire in northwestern Syria, where hostilities have displaced half a million people in the past two months. Russia-backed forces of the Syrian regime have upped their deadly bombardment of the last major opposition bastion in the northwest since December, chipping away at its southern edge. The violence in the extremist-ruled region of Idlib has forced 520,000 people out of their homes since the start of December, in one of the biggest upheavals in the nine-year war. The aid groups -- including the Norwegian Refugee Council, Save the Children, Care and the International Rescue Committee -- labelled the situation a "humanitarian catastrophe". They called for "an immediate cessation of hostilities in addition to immediate access to safety for the millions of civilians currently under fire". The latest wave of people fleeing follows 400,000 others who were displaced by an earlier round of fighting in Idlib last year. Many have fled north towards the border with Turkey, where camps are overcrowded and thousands more have instead set up haphazard tents in olive groves. Jan Egeland, head of the Norwegian Refugee Council, warned that the newly arrived were running out of options as to where to go. "Camps are hosting five times their intended occupancy and rental prices have skyrocketed in towns in the north west," he said. "We are calling on Turkey to let these terrified families seek safety either across the border or in areas Turkey controls in Syria." Andrew Morley, the head of World Vision International, said children were sleeping in flooded fields, and some families were even burning their clothes to stay warm. "The exodus of people is staggering, and tens of thousands more are joining them every day," he said. The recent of violence has killed around 300 civilians, the Syrian Observatory for Human Rights war monitor says. The World Health Organization said on Monday that the violence had forced 53 medical facilities in northwest Syria to close in January and warned of "critical health threats" to fleeing civilians. Syrias war has killed more than 380,000 people and displaced more than half the countrys pre-war population since starting in 2011 with the brutal repression of anti-regime protests.