Top UN officials said Saturday they were "appalled" by escalating violence in Syria, and urged immediate access to Aleppo, where government forces are waging a ferocious assault to retake rebel-held districts.

"The United Nations is extremely saddened and appalled by the recent escalation in fighting in several parts of Syria and calls on all parties to cease indiscriminate attacks on civilians and civilian infrastructure," the UN's humanitarian coordinator for Syria Ali al-Zaatari and regional humanitarian coordinator Kevin Kennedy said.