"What we are seeing, in Eastern Ghouta and elsewhere in Syria, are likely war crimes, and potentially crimes against humanity," UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said on Friday in a meeting on the situation in Eastern Ghouta at the 37th session of the UN Human Rights Council in Geneva.

"Civilians are being pounded into submission or death. The perpetrators of these crimes must know they are being identified, that dossiers are being built up with a view to their prosecutions, and that they will be held accountable for what they have done," he added.

The UN rights chief said that despite the five-hour pause announced by Russia to allow medical and humanitarian aid, airstrikes and ground-based strikes had continued.

"Moreover, the humanitarian agencies have made it very clear that it is impossible to deliver aid during a five-hour window as it can take up to one day to simply pass checkpoints," he said.

"Syria must be referred to the International Criminal Court. Attempts to thwart justice, and shield these criminals, are disgraceful," he said.

Eastern Ghouta, a Damascus suburb, has been under siege for the last five years, and humanitarian access to the area, which is home to some 400,000 people, has been completely cut off.

In the past eight months, Bashar al-Assad regime forces have intensified their siege of Eastern Ghouta, making it nearly impossible for food or medicine to get into the district and leaving thousands of patients in need of treatment.

Hundreds have been killed by regime airstrikes in recent days.

Last Saturday, the UN Security Council adopted a resolution calling for a 30-day cease-fire in Syria without delay.