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 conflict in which Russian-backed regime forces have launched a major offensive against rebels in and around Aleppo

"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.

"We can't negotiate with the terrorists of the Islamic State, but if we were able to reach an agreement between anti- and pro-Assad forces on a kind of no-fly zone, in the sense of a sanctuary for the many refugees, then this would save many lives and aid the political process about Syria's future," she said.

Turkey, which has already taken in an estimated 2.7 million Syrian refugees, has been calling for a secure zone within Syria where the displaced could find refuge.