UN Secretary-General Antonio Guterres said it is “high time to stop this hell on Earth” in Syria’s eastern Ghouta region, where hundreds of civilians have died over the past month.
He appealed on all sides to abide by a 30-day nationwide ceasefire, which the 15-member Security Council unanimously voted on over the weekend.
The council came to an agreement after days of Russia – Syria’s key military ally in the civil war – refusing to agree unless the ceasefire excluded fighters from IS and the al Qaeda-linked Levant Liberation Committee.
Mr Guterres said he welcomed the resolution but it is “only meaningful…if effectively implemented”.
“That is why I expect the resolution to be immediately implemented and sustained,” he told the opening of the 37th session of the UN Human Rights Council in Geneva.
Meanwhile, another top UN official criticised the lack of action in conflicts around the world, including Syria.
Zeid Ra’ad Al Hussein, the UN’s human rights chief, said: “Eastern Ghouta, the other besieged areas in Syria, Ituri and the Kasais in the (Democratic Republic of Congo), Taiz in Yemen, Burundi, Northern Rakhine in Myanmar have become some of the most prolific slaughterhouses of humans in recent times because not enough was done, early and collectively, to prevent the rising horrors.”
He said Security Council members who had vetoed action bore “responsibility for the continuation of so much pain.”
“It is time, for the love of mercy, that China, Russia and the United States end the pernicious use of that veto,” he told the rights council.
“We have every reason to remain cautious,” he added.
“The resolution must be viewed against a backdrop of seven years of failure to stop the violence, seven years of unremitting and frightful mass killing.”
The European Union’s diplomatic chief, Federica Mogherini, also demanded the ceasefire be implemented “immediately” and “without delay” as she arrived for a meeting of EU foreign ministers in Brussels.
Their remarks came as the Syrian government kept up its bombardment of Eastern Ghouta, near the capital Damascus, on Monday, killing at least 10 civilians, including nine members of the same family, the UK-based Syrian Observatory for Human Rights group said.
Opposition activists claimed it was a chlorine gas attack, saying a three-year-old had died of asphyxiation.
The Syrian American Medical Society said 16 patients in its hospital had symptoms indicating they were exposed to chemical compounds.
Assad’s regime has continually denied using chemical weapons but has been accused of carrying out several chlorine gas attacks in recent weeks, including a further two in January in eastern Ghouta.
Since 18 February more than 500 civilians have been killed in bombings by the Assad regime and its Russian allies on the enclave east of Damascus, the monitoring group said.
More from Eastern Ghouta
Syria regime ‘in deadly chlorine gas attack on civilians’
Syria conflict: Fresh airstrikes on eastern Ghouta as UN set to vote on ceasefire
The area, home to 400,000 people, is controlled by Islamist and jihadist fighters as well as rebel fighters.
It is the last major area near the Syrian capital which remains under rebel control.