Nearly 3.5 million civilians in Syria have virtually no access to desperately-needed humanitarian aid and people are dying needlessly every day as violence and extremism escalate and the risk of sectarian violence grows, United Nations chief Ban Ki-moon has said.
In a report to the UN Security Council, the secretary general criticised both the Syrian government and rebel forces for their failure to implement a UN resolution adopted in February.
The resolution is aimed at eliminating obstacles to deliver food, medicine and other essentials to millions of civilians caught up in the three-year civil war that has killed more than 150,000 people.
It demanded that all parties, especially the Syrian government, promptly allow safe access for humanitarian aid across conflict lines and borders and called on both sides to immediately lift sieges of populated areas and stop depriving civilians of food.
It also demanded a halt to indiscriminate shelling and aerial bombardment, including the use of barrel bombs in populated areas and the withdrawal of all foreign fighters from Syria.
"Two months since the adoption of Security Council resolution 2139, none of the parties to the conflict have adhered to the demands of the council," Mr Ban said.
"The security situation is deteriorating and humanitarian access to those most in need is not improving. Thousands of people are not getting the medical care, including life-saving medicines, that they need. ... Almost 3.5 million civilians remain largely without access to essential goods and services."
The secretary general said the council's call on both sides to lift the sieges of populated areas "has not been heard and I consider it shameful that nearly a quarter of a million people are being deliberately forced to live under such conditions".
Mr Ban reported indiscriminate aerial bombardments by the government and mortar attacks and shelling by armed opposition groups, and said "foreign fighters continue to support all sides to the Syrian conflict, including extremist groups, armed opposition groups and the government".
He said the Security Council, which has been deeply divided over Syria but united to adopt the February resolution, "must take action to deal with these flagrant violations ... of international law".
The resolution threatened no sanctions - at the insistence of Russia, which is Syria's closest ally - but it did express the council's intention to take "further steps" if its demands are not fulfilled.
Diplomats have talked of a new humanitarian resolution and France has drafted, but not yet circulated, a resolution to refer the Syrian conflict to the International Criminal Court. But winning approval in the council for any resolution with teeth will almost certainly prove very difficult.
The secretary general said denying medicine to the sick and wounded was a clear violation of international law "yet medicines are routinely denied to those who need them, including tens of thousands of women, children and elderly".
He again called on the Syrian government to streamline procedures for aid convoys and grant blanket approvals to reach all those desperately in need of assistance.
About 9.3 million people - more than 6.5 million displaced by the fighting - are in need of urgent humanitarian assistance within Syria, including the 3.5 million in areas that are difficult or impossible to reach and have virtually no access to aid, Mr Ban said.
He said the UN World Food Programme delivered food to 4.1 million people in March, up from 3.7 million in February. But he said aid only reached 34 of the 262 locations identified as being hard to reach or besieged, and humanitarian agencies are facing increasing challenges to deliver assistance.