More people have fled fighting in Syria over the past 10 weeks than at any other time in the nearly nine-year-old conflict and the city of Idlib, where many are sheltering, could become a graveyard if hostilities continue, two United Nations agencies said on Tuesday.

Syrian government forces are shelling their way northwards, backed by Russian air raids, driving people towards the Turkish border as they try to seize remaining rebel strongholds near Idlib and Aleppo.

Turkey, which backs the rebels and is fearful of additional refugees, has retaliated militarily, with displaced civilians caught in between.

"It's the fastest-growing displacement we have ever seen in the country," Jens Laerke from the UN Office for the Coordination of Humanitarian Affairs (OCHA) said, adding that nearly 700,000 people have fled since December, mostly women and children.

Of Syria's 17 million people, 5.5 million are living as refugees in the region, mostly in Turkey, and a further six million are uprooted within their own country.

Civilians are struggling to find shelter, amid harsh winter conditions with snow, rain and wind from Storm Ciara. Mosques are full and makeshift camps are overcrowded, said Andrej Mahecic, a spokesman for the UN refugee agency.

"Even finding a place in an unfinished building has become nearly impossible," he told journalists in Geneva, describing the humanitarian crisis as "increasingly desperate".