"Such indiscriminate attacks are unacceptable," Stephen O'Brien said in a press conference concluding a three-day visit to Syria.
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More than 50 civilians were killed on Sunday in air strikes and rocket fire on an opposition stronghold east of Damascus and in rebel fire on the capital.
"This is a tragic reminder of the urgency of finding a political solution and securing a nationwide ceasefire," O'Brien said.
During his visit, O'Brien met with residents of Waer, a neighbourhood in the central city of Homs where rebels and the regime recently entered a UN-administered ceasefire. He also met Syrian Foreign Minister Walid Muallem and other senior officials.
O'Brien said the UN had been able to deliver lifesaving aid to the district for the first time since January.
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According to O'Brien, nearly three-quarters of Syria's six million internally displaced people have no access to drinking water, and two million children are out of school. "This situation is unacceptable. A blot on our collective conscience," O'Brien said.
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