A war monitor said Russian warplanes hit a school basement in eastern Ghouta, killing at least 15 children. In New York, the UN human rights chief was blocked from addressing the Security Council on the carnage in Syria.
The children and at least two women were killed when an airstrike hit the school basement they were sheltering in, the Syrian Observatory for Human Rights reported on Monday.
An official with the Observatory said planes believed to be Russian hit the school shelter, which was being used by civilians.
"Usually regime planes do not fly at night, but Russian warplanes do," said activist Nour Adam.
Fifty-two people were also wounded in the strike, the Britain-based Observatory reported. Syrian government forces backed by Russian air power began a major operation to retake eastern Ghouta, on the outskirts of the Syrian capital Damascus, a month ago.
Observatory head Rami Abdurrahman told the German press agency dpa that the death toll was likely to go up as some of the injured had serious wounds.
Local activists described the scene as "horrific" and said bodies of small children were everywhere.
"Those people were hiding in the basement of the school thinking schools will not be hit, but for this tyrant Syrian President Bashar al-Assad and his allies the Russians, there are no safe areas inside eastern Ghouta, all for them are terrorists, even the children," said Abdurrahman.
Since the assault was launched, Syrian government forces have advanced deep inside the enclave and split it up, triggering a mass exodus from the area, which UN Secretary General Antonio Guterres has called "hell on earth."
Roughly 50,000 civilians have fled the embattled region to government-controlled areas since Thursday alone, according to estimates from the Observatory.
Rocket strikes Damascus
Separately, at least 35 people were killed in a rocket attack on capital Damascus. Syrian state media said opposition fighters, who have faced major reverses in besieged eastern Ghouta over the past month, were responsible for the attack.
The "terrorist" attack on a busy market place was one of the deadliest targeting the Syrian capital since the start of the civil war in 2011.
The rocket landed in the Kashkol neighborhood close to rebel-held parts of eastern Ghouta.
The Syrian army says rebels frequently fire rockets and mortar shells targeting civilians in the state-controlled parts of Damascus. Rebels deny targeting civilians.
UN rights chief blocked from speaking
At an informal meeting of the UN Security Council on Monday, UN Human Rights Chief Zeid Ra'ad al-Hussein said that the Syrian government's five-year siege of eastern Ghouta involved "pervasive war crimes," use of chemical weapons and starvation as a weapon of war.
Zeid was blocked from addressing a formal council meeting by a Russian procedural maneuver, but he delivered his hard-hitting speech to an open meeting anyway, decrying "mind-numbing crimes" committed by all parties in Syria using "unlawful methods of warfare."
He was particularly critical of the Syrian government, and singled out President Bashar al-Assad.
"When you are capable of torturing and indiscriminately killing your own people, you have long forfeited your own credibility," said Zeid dismissing Assad's claim that his government made every effort to protect civilians.
Zeid also made a point of saying that "those who have perpetrated and are still perpetrating these mind-numbing crimes committed in Syria must be made to answer before a properly constituted court of law."
He once again urged the council to refer Syria to the International Criminal Court. That however remains highly unlikely, because both Russia and China vetoed a resolution backed by more than 60 countries in 2014 that would have referred the Syrian conflict to the ICC.
Zeid had been scheduled to speak at an open council meeting on Monday. But when it started, Russian Deputy Ambassador Gennady Kuzmin protested that it was a matter for the Human Rights Council in Geneva, not the Security Council.
Francois Delattre, France's UN Ambassador, criticized Russia for refusing any discussion of human rights in the Security Council.
Britain's deputy UN ambassador Jonathan Allen said Russia "doesn't want the truth of ... the appalling human rights abuses taking place."
av/se (Reuters, AP, AFP, dpa)