The United Nations aid chief on Monday condemned attacks on civilians in and near the Syrian capital as "unacceptable," a day after more than 50 were killed.

"Such indiscriminate attacks are unacceptable," Stephen O'Brien said in a press conference concluding a three-day visit to Syria. 

More than 50 civilians were killed on Sunday in air strikes on an opposition stronghold east of Damascus and in rebel fire on the capital.