Assistant Secretary-General for Humanitarian Affairs Ursula Mueller told the UN Security Council in a video briefing from Amman, Jordan, that there have been no UN aid convoys to besieged areas in July and just one a week to hard-to-reach areas, meaning just over 120,000 people got help this month.
Mueller blamed the Syrian government, armed groups, insecurity and fighting.
Other UN partners delivered aid to some hard-to-reach areas, where an estimated four million people live. The UN says 540,000 people in 11 locations are still under siege, mostly by Assad's forces.
Mueller said the UN continues to see a reduction in violence in some areas since Russia and Iran, both supporters of Syrian President Bashar al-Assad, and Turkey, which backs rebels fighting Syrian government forces, agreed on a plan in May to establish four "de-escalation" zones in Syria.
But "despite reductions in violence, we have not been able to noticeably increase our reach," she said.
In a letter to the Security Council, ambassadors from 12 Western and Arab nations and the European Union, who attend weekly meetings in Geneva on humanitarian aid to Syria, said they remain "extremely concerned" that the UN is being excluded from sending convoys to besieged and hard-to-reach areas.
"This trend has worsened significantly in recent months," said the letter, obtained by The Associated Press.
"Only two UN supported convoys have been able to access territory besieged by the Syrian authorities since April."
The ambassadors urged the Security Council to underline to Syria its obligation under international law to allow humanitarian aid deliveries - and to take action given its pledge to "take further steps in the case of non-compliance".
Britain's UN Ambassador Matthew Rycroft told reporters: "We're not asking for humanitarian access as a favour. We're asking for it because it's a legal and moral obligation. It's an obligation under successive Security Council resolutions."
Al Jazeera's Rosalind Jordan, reporting from New York, said that a new UN resolution is highly unlikely, according to China's UN ambassador Liu Jieyi, who is acting president of the Security Council.
"[He] told reporters that members of the council are very worried about the humanitarian situation in Syria and that they're looking for a way to try to make it easier for humanitarian deliveries of food and medicine to actually get to the people who need it, but there aren't any new initiatives on the table," said Jordan.
"There are already a number of resolutions calling for increased aid deliveries."
Over 200,000 have fled Syria's Raqqa since April 1
Mueller said that over 200,000 civilians had fled their homes in Syria around Raqqa, the de facto capital of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, since April 1, including over 30,000 displaced just this month as US-backed Syrian fighters try to expel the group.
Mueller told the UN that "an estimated 20,000 to 50,000 people remain inside Raqqa city, which is now encircled, and their situation is perilous - there is no way for them to get out."
For those displaced in Raqqa province, she said humanitarian conditions are very difficult with temperatures now approaching 50 degrees Celsius. The UN also has serious concerns over their protection, particularly over their freedom of movement outside the camps many now live in, she said.
Leaving the city of Raqqa remains "extremely difficult due to the presence of mines and other unexploded ordnance, as well as shelling, sniper activities and airstrikes," Mueller said.
"As military operations continue, our concern is further civilian casualties," she said, especially since fighters from the ISIL group have allegedly used civilians as human shields.
US-backed Syrian fighters have captured almost half of Raqqa from ISIL, but the push into the northern city has slowed due to the large amounts of explosives planted by the armed group.
Mueller said the UN and its partners are ready to support the people of Raqqa as soon as security allows it and they can gain access.
"The health situation, particularly the low availability of trauma care services, is a major concern in view of the intense fighting and shifting front lines," she said. "We continue to engage with relevant parties and actors on the ground ... but a lot more needs to be done."
Source: Al Jazeera and news agencies