UN humanitarian chief Valerie Amos told the Security Council on Wednesday only 12 percent of Syrians in "hard to reach areas" have received aid, despite a UN resolution, diplomats said.
Instead of alleviating the misery of more than nine million Syrians who need urgent assistance, the humanitarian crisis has worsened since Resolution 2139 was adopted in February.
Amos said all parties to the conflict were guilty of violations, according to a diplomat who attended the closed door briefing.
Only 15 percent of locations identified as in need of aid had been reached and only 12 percent of Syrians in "hard to reach areas" had received assistance, Amos told the Council.
"Far from getting better, the situation is getting worse," the diplomat quoted her as saying.
Amos demanded security guarantees so that aid convoys could cross conflict and border lines, telling the 15-member Council that there was no time to wait for travel permits.
The UN estimates that 3.5 million people in Syria live in hard to reach areas and 250,000 are besieged, the majority by Syrian security forces but some also by armed opposition groups.
It is Amos's second briefing since the resolution was passed and comes a week after UN chief Ban Ki-moon said aid access had not improved, blaming both sides but singling out the government.
Under Resolution 2139, Security Council members are committed to "take further steps" if the resolution is being implemented.
But to introduce targeted sanctions a new resolution would be needed, which Russia, a key ally of Damascus, would veto.
Russia and China have vetoed three previous Security Council resolutions on Syria since the conflict began three years ago.
Russian ambassador Vitaly Churkin told the Council that the main obstacles were "terrorist activities," diplomats said.
French ambassador Gerard Araud accused Churkin of parroting the words of the Syrian government and Russia of ignoring reality.
To pressure Damascus and embarras Moscow, Western powers want to refer Syria to the International Criminal Court for war crimes.
Amnesty International on Wednesday demanded that the Security Council take action, including the threat of targeted sanctions, against those flouting the terms of the February resolution.
"The humanitarian situation in Syria is beyond catastrophic. More than two months after a UN resolution to alleviate the suffering of civilians and end war crimes was adopted, the situation there has only worsened," said Jose Luis Diaz, head of the London-based rights group's UN office in New York.
"If the Security Council is to salvage what credibility it has left on Syria it has to ensure its unanimous decision is respected, including by making good on its intention to take further steps to get the different parties to comply."
Additional measures, including sanctions, must be taken against those responsible for violating the resolution, Amnesty said.