UN Secretary-General Ban Ki-moon said Thursday that humanitarian access in Syria was worsening, blaming the government in particular but also armed rebels in a third report on the matter.
Ban said Damascus was still stopping aid convoys from crossing into Syria from Iraq, Jordan and Turkey in violation of Security Council Resolution 2139 adopted in February.
Cross-border access would deliver aid to millions of Syrians in regions controlled by the armed opposition.
Ban called on the Council to "urgently consider what steps it will now take to secure compliance with its demands," including lifting sieges and opening cross-border points.
The Council is due to debate the report on May 29.
For now, the 15 members have failed to reach an agreement with Russia providing diplomatic cover to Damascus.
The report relating to the period from April 22 to May 19 said more than 3.5 million Syrians continue to have "unpredictable and woefully inadequate" access to humanitarian assistance.
This includes 241,000 people who live in besieged areas -- 196,000 of them in areas besieged by government forces and 45,000 in areas besieged by opposition forces, Ban said.
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"Access to government and opposition areas has dropped significantly since the last reporting period," according to the report.
A major factor has been a new procedure introduced May 5 requiring the sealing of trucks carrying humanitarian supplies, intended to facilitate passage at checkpoints.
"The lack of understanding of the new procedure by security officials at checkpoints has resulted in delays," it said.
The report said the situation on the ground has got worse and that food deliveries slowed considerably during May.
"Parties to the conflict, particularly the government of Syria, continue to deny access for humanitarian assistance in a completely arbitrary and unjustifiable manner," it said.
"I remind the Council that intentionally using starvation as a weapon of war is a gross violation of international humanitarian law," the report said.
On Thursday, China and Russia vetoed a Western attempt to refer both sides in Syria to the International Criminal Court, spotlighting international paralysis in how to resolve the war.
Western powers want the Council to adopt a resolution enforcing convoy access at four border crossings.
Australia and Luxembourg are drafting a text due to be submitted in the coming weeks.
Russia, which has provided diplomatic cover to its ally Damascus, and China have so far blocked four resolutions on Syria since the brutal war broke out more than three years ago.