Here are some of their reactions:

- 'No words' -

The bloodshed prompted the UN children's agency UNICEF to issue a blank statement.

"No words will do justice to the children killed, their mothers, their fathers and their loved ones," UNICEF's Geert Cappelaere said in a footnote.

"We no longer have the words to describe children's suffering and our outrage. Do those inflicting the suffering still have words to justify their barbaric acts?"

- 'Flagrant war crimes' -

Amnesty International accused the Syrian government of intentionally targeting its own people.

"People (in Eastern Ghouta) have not only been suffering a cruel siege for the past six years, they are now trapped in a daily barrage of attacks that are deliberately killing and maiming them, and that constitute flagrant war crimes," Amnesty researcher Diana Semaan said in a statement.

"It is imperative for the (UN) Security Council to send a strong message that there will be no impunity for those who commit war crimes and crimes against humanity."

- 'Children dying by the hour' -

Save the Children called the situation "absolutely abhorrent".

"The bombing has been relentless, and children are dying by the hour. These families have nowhere left to run -- they are boxed in and being pounded day and night," said Sonia Khush, the organisation's Syria response director.

"We urgently need a ceasefire so that the killing and maiming stops."

- 'Humanitarian catastrophe' -

CARE International said the escalating violence has made it impossible for humanitarian agencies to reach people in need.

"Civilians are deprived of food and medicine and are facing hunger and death" said Wouter Schaap, the group's Syria director.

"If a ceasefire is not reached now, we will be facing a humanitarian catastrophe."

- 'Cannot let history repeat' -

The International Committee of the Red Cross (ICRC) said "this cannot go on".

"Families trapped, with no safe place to hide from shelling. Dozens of mortars in Damascus cause civilian casualties and spread fear. We cannot let history repeat itself," the group said on Twitter.