UN Secretary-General Ban Ki-Moon has demanded that the Security Council take action in Syria on violations of international law.
He reported to the body that none of the warring parties was adhering to UN demands for aid access.
The comments came in his second monthly report to the 15-member council on the implementation of a resolution demanding great humanitarian aid access in Syria.
Mr Ban said: "The Security Council must take action to deal with these flagrant violations of the basic principles of international law."
He did not specify what measures the council should take.
Two months after the council achieved rare unity to unanimously approve a resolution demanding rapid, safe and unhindered aid access, including across borders, Mr Ban said the situation "remains an extremely challenging environment in which to work".
In that resolution, the Security Council expressed "its intent to take further steps in the case of non-compliance" with the resolution.
However, diplomats said Russia was unlikely to agree to any action, such as sanctions, if Syria's government was found to be at fault.
Mr Ban issued a similar report last month.
Earlier, UN Humanitarian Chief Valerie Amos said hope for a political deal to end Syria's war was fading and it was getting harder every day to send aid to millions of trapped and displaced civilians.
Last month, the United States and European delegations said Mr Ban's reporting had shown the government was largely responsible for the lack of aid access, which the government denies.
They said any problems caused by rebel groups were the work of Islamist fighters linked to al-Qaeda.
The situation is getting worse, Mr Ban said.
"Thousands of people are not getting the medical care, including life-saving medicines, that they need," he reported.
"Medical supplies, including life-saving medicines and vaccines, and equipment for the wounded and the sick are commodities privileged throughout the Geneva Conventions.
"Denying these is arbitrary and unjustified, and a clear violation of international humanitarian law.
"Yet, medicines are routinely denied to those who need them, including tens of thousands of women, children, and elderly."
Mr Ban said that nearly 3.5 million people were largely without access to essential goods and services due to the Syrian civil war, which is now in its fourth year.
The Security Council is due to discuss the report next week.
Separately, after 15 days without access, United Nations Relief and Works Agency is to be allowed to resume food distribution in Yarmouk refugee camp in Syria.