(CNN) Waves of deadly airstrikes against rebel-held areas in Syria have made this week "one of the bloodiest periods of the entire conflict," the UN human rights chief said Saturday.

Opposition-controlled areas in Syria's northwestern province of Idlib and Eastern Ghouta, outside the capital, Damascus, have suffered intensified bombardment by what observers say are Russian-backed Syrian government forces.

UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein called for urgent international action to halt the bloodshed after reports of 277 civilian deaths between Sunday and Friday. More than 800 civilians also were reported injured, the UN human rights office said.

White Helmets rescuers evacuate a victim of an airstrike Thursday in the rebel-held enclave of Hazeh.

"The past week has been one of the bloodiest periods of the entire conflict, with wave after wave of deadly airstrikes leading to civilian casualties in areas of Eastern Ghouta and Idlib," Zeid said in a strongly worded statement.

"The no-holds-barred nature of this assault is evidenced by reports that at least nine medical facilities, six of them in Idlib and three in Eastern Ghouta, were hit by airstrikes. Even by Syria's atrocious standards, these are exceptionally deplorable developments."

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