Millions of Syria's children are at risk of malnutrition and face severe food shortages, international aid organisation Save the Children has warned.

It said four million Syrians - more than half of them children - were unable to get enough food.

Thousands are trapped in battle zones around Syria's major cities, cut off from access to all but the bare minimum needed to survive.

Shortages have been made even worse by an explosion in prices for basics, the charity said, adding that one in 20 children in and around Damascus is severely malnourished.

Ever since the conflict erupted in March 2011, aid groups have called on both the government and the rebels to give them access to civilians trapped by the fighting.

But their calls have consistently been ignored.

"The world has stood and watched as the children of Syria have been shot, shelled and traumatised," said Save the Children Middle East director Roger Hearn.

"The conflict has already left thousands of children dead and is now threatening the means of others staying alive."

At the United Nations, the head of the World Food Programme (WFP) demanded on Monday that any ceasefire agreement include access for aid workers.

Ertharin Cousin said that any deal that envisions a cessation of hostilities so chemical weapons experts can travel would include conflict areas which WFP has been unable to reach.

It is currently feeding three million people inside Syria and 1.2 million in neighbouring countries.