UN human rights chief Zeid Ra'ad Al Hussein accused Syrian regime forces of carrying out "no-holds-barred" military offensives after a spike in violence left hundreds of civilians dead.

Calling for "urgent international action" to protect civilians, Zeid also slammed what he called an "epic failure of global diplomacy" to end the seven-year-old war.

"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 statement.

According to the Geneva-based UN Human Rights Office, at least 277 civilians were killed last week including 230 in air strikes by Syrian regime and allied forces, the statement said.

Another 812 civilians were injured, it said.

"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 air strikes," the statement added.

UN 'paralysis'

"After seven years of paralysis in the Security Council, the situation in Syria is crying out to be referred to the International Criminal Court, as well as for a much more concerted effort by states to bring peace," the statement said.

"The conduct and management of this war has been utterly shameful from the outset, and the failure to end it marks an epic failure of global diplomacy."

The UN Security Council is currently examining a draft resolution calling for a 30-day ceasefire in all of Syria to enable the urgent delivery of humanitarian aid, according to a text seen by AFP.

The draft resolution proposed by Sweden and Kuwait also calls for an immediate end to all sieges including in Eastern Ghouta.

The Council is expected to begin debate on the resolution Monday.

Russia's position on the resolution was not known.

Moscow has repeatedly blocked action in the council that would put pressure on Syrian regime leader Bashar al Assad.

Earlier this week the Council failed to back an appeal by UN aid officials for a month-long pause in fighting after Russia, a long-time ally of the Syrian regime, rejected the proposal.

Russian Ambassador Vassily Nebenzia said it was "not realistic" to impose a ceasefire because armed groups fighting Assad's forces were unlikely to uphold it.

The United States earlier this week accused Russia of delaying the adoption of a UN Security Council condemnation of reported chlorine gas attacks in Syria that left many injured in recent days, including children.

US Ambassador Nikki Haley told the council that there was "obvious evidence from dozens of victims" to corroborate the chlorine attacks in opposition-held Eastern Ghouta.

"Now we have reports that the Assad regime has used chlorine gas against its people multiple times in recent weeks, including just yesterday," Haley said.

Regime eases strikes on opposition enclave

Syrian regime air strikes on the opposition enclave of Eastern Ghouta eased on Saturday after five days of heavy bombardment that killed more than 240 civilians, a monitoring group said.

The Britain-based Syrian Observatory for Human Rights said five people were wounded in artillery strikes but that the intensity of bombardment targeting the Eastern Ghouta region near the capital had lessened from Friday night.

"There have (since) been practically no air raids and even artillery fire is less intense", Observatory head Rami Abdel Rahman said, although warplanes had struck areas near Douma, the region's main town.

Opposition shelling triggered a fire at a power plant in the capital, the SANA news agency said.

The sharp drop in violence came as regime forces and their allies were targeted in major Israeli air raids Saturday after an Israeli warplane crashed after being fired on by Syrian air defences.

Five straight days of bombardment by regime warplanes and artillery from Monday to Friday killed more than 240 civilians, including 60 children, and wounded 775 people, the Observatory said.

The offensive had trapped thousands of families in makeshift bomb shelters and overwhelmed rescue workers, but on Saturday life in Eastern Ghouta appeared to be returning to normal in some areas.

Some shops reopened in Douma, Erbin and Hammuriyeh, and residents were seen sweeping up rubble outside their homes as others salvaged what they could from destroyed houses, AFP correspondents said.

Syria's war has killed more than 400,000 people and displaced millions since it began in March 2011 with the brutal repression of anti-regime protests.