"The latest offensive by the Syrian regime against Aleppo -- supported by Russia -- has made the suffering of the civilian population yet worse," the two leaders agreed in a telephone conversation, according to a statement released by Merkel's office.
Russia said today that it would continue its Syrian air campaign in the face of warnings from US Secretary of State John Kerry that Washington would pull the plug on any more talks unless Moscow stopped the bombing of the besieged city.
UN Secretary-General Ban Ki-moon on Wednesday labelled bombings that hit the two main hospitals in Aleppo's rebel district "war crimes".
"The repeated flagrant violations against humanitarian international law which have been reported are unacceptable. A ceasefire is more urgent than ever," Merkel and Erdogan agreed in their conversation, which also touched on the fight against the Islamic State group.
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Turkey launched an offensive dubbed "Euphrates Shield" on August 24 to drive IS jihadists and Kurdish rebel fighters away from its southern frontier.
Ties between Russian President Vladimir Putin and Erdogan were badly shaken in November, when Turkish forces shot down a Russian bomber they said had strayed into their airspace from Syria.
But Moscow and Ankara have since removed their relations from the deep freeze with promises of closer cooperation.