It comes amid the increasing number of strikes targeting the rebel-held enclave of Eastern Ghouta
The UN Security Council is expected to vote on a new plan for a Syria ceasefire today.
The resolution, which has been drafted by Sweden and Kuwait, would allow for a 30-day break in the fighting to deliver humanitarian aid.
It remains unclear whether Russia - a key ally to Syrian President Bashar al-Assad - will use its veto to block the deal.
It comes amid the increased violence in the rebel-held Eastern Ghouta enclave in recent days.
Eastern Ghouta - which is around 15 kilometres from Damascus city centre - has been besieged by the Syrian army since 2013, and is now one of the few remaining areas in the country still under rebel control.
The last several weeks have seen the area targeted by Russia-backed Syrian government forces.
More than 300 civilians - including dozens of children - are reported to have been killed since Sunday, with many others injured.
In a statement yesterday, UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein called for an end to the "monstrous annihilation" in Eastern Ghouta.
He said: “These are hundreds of thousands of civilians who have been trapped for over five years under siege, suffering deprivation of their most basic needs, and are now facing relentless bombing.
"How much cruelty will it take before the international community can speak with one voice to say enough dead children, enough wrecked families, enough violence, and take resolute, concerted action to bring this monstrous campaign of annihilation to an end?”
Aid agencies and activists have also called for action, with Amnesty International saying the long-running siege and current barrage of airstrikes "constitute flagrant war crimes".
The International Red Cross has called for urgent access to Eastern Ghouta to deliver humanitarian aid.
The conflict in Syria has seen more than 300,000 people killed and millions more displaced since it began in early 2011.