In July, not a single aid convoy was allowed to reach civilians in the 11 besieged areas, UN deputy aid chief Ursula Mueller told the Security Council.
While the fighting has eased in Daraa governorate and other areas, "the humanitarian and protection situation remains extremely difficult for civilians in many parts of the country," Mueller said.
Military operations have restarted in rebel-held eastern Ghouta, near Damascus, and in the capital's Jobar neighbourhood, among other areas, she said.
In Idlib, infighting among opposition and rebel groups have forced some aid agencies to suspend their work.
Mueller accused the Syrian government of blocking aid convoys but said armed groups were also obstructing access, especially in Idlib and the eastern governorates of Syria.
"This means that, despite reductions in violence, we have not been able to noticeably increase our reach," said Mueller, the UN assistant secretary-general for humanitarian affairs.
Over the past two months, just over a third of the one million Syrians living in besieged and hard-to-reach areas received aid, Mueller said.
Britain, France and other countries that make up the international humanitarian task force for Syria have written to the Security Council to express their concern about the failure to ramp up aid deliveries.
British Ambassador Matthew Rycroft said that in the last three months only two areas besieged by Syrian forces have received aid.
"We are not asking for humanitarian access as a favour. We are asking for it, because it's a legal and moral obligation," Rycroft told reporters.
Syrian Ambassador Bashar Jaafari said Islamist armed groups were to blame for blocking aid deliveries and rejected UN accusations that the government was uncooperative.
"These reports are used to slander the Syrian government, to apply pressure on the Syrian government and its allies in the fight against terrorism," Jaafari told the council.
He charged that aid convoys "often go to armed terrorist groups instead of reaching the Syrian people who need them."
An estimated 20,000 to 50,000 people remain trapped in Raqa as a US-backed alliance battles to oust the Islamic State group from the northern city.
"There is no way for them to get out," said Mueller, citing the danger of mines, unexploded ordnance, shelling, snipers and airstrikes.
Dozens of civilians have been killed and injured by airstrikes and shelling in July and over 30,000 people have been displaced since July 1, she said.
Over 200,000 Syrians have fled fighting in Raqa since April 1, according to UN figures.
A polio vaccination campaign was launched this week in Raqa targeting 450,000 children in Raqa and Deir Ezzor. So far 27 polio cases have been confirmed.
More than 330,000 people have lost their lives in Syria since the country's conflict broke out in March 2011 with anti-government protests.