UN and Arab League envoy Lakhdar Brahimi has begun his first mission to Syria with a visit to Cairo, where he acknowledged the challenges he faces.
Mr Brahimi said it would be a "very difficult mission", adding that he would visit Damascus in a few days.
He replaces former envoy and UN chief Kofi Annan, who resigned over division in the UN Security Council.
Violence in Syria has continued with government jets targeting opposition-held areas of Aleppo, activists said.
The attacks come a day after dozens of people, including many troops, were killed by a car bomb in Aleppo which state TV reported targeted two hospitals and a school.
The Free Syrian Army said it had carried out the attack because the facilities were being used by government troops.
Reports from Syria are almost impossible to verify because of severe restrictions on foreign journalists in the country.
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