Colville said there may be as many as 50,000 civilians trapped in the town of Yabroud, a rebel-held area near Lebanon's eastern border. He said there have been "numerous" aerial attacks on the town, electricity was cut off and medical supplies were running low.
"We are deeply concerned that the attack on Yabroud may follow the pattern of previous attacks on cities and towns across Syria where government aerial bombardments were indiscriminate and disproportionate," he said in a statement from Geneva.
Such acts are a violation of international law and Syria is reminded that Pillay already this week suggested the situation in Syria should be referred to the International Criminal Court, he said.
"We are closely watching the situation unfold in Yabroud and urge all sides to respect their obligations under international law," he said in a statement.
"Syria itself is crumbling," he said in a statement.