German Chancellor Angela Merkel on Wednesday described the humanitarian situation in Syria as "intolerable" and reiterated her call for a no-fly zone to protect civilians.

"The current situation in Syria remains intolerable," she said about the Russia-backed regime offensive on Aleppo that has sent thousands of refugees fleeing from northern Syria.

"It would be helpful if there were an area in Syria where none of the parties to the conflict launched aerial attacks," Merkel told German parliament.