Aid groups say that more than a million Syrians now live under siege without access to sustained humanitarian assistance.
"Our findings show a clear pattern of obstruction and manipulation by Syrian authorities, who ensure that approved aid rarely reaches its intended targets, and when it does, it is wholly inadequate, "said Elise Baker, the organisation's lead Syria researcher.
Humanitarian access was meant to accompany a ceasefire brokered in December by Russia and Turkey. But in January, Jan Egeland, a senior UN adviser, blamed the stoppage on a "complete, hopeless, bureaucratic quagmire".
While the UN usually works at the invitation of the governments that host it, critics say the organisation's reliance on the approval of Assad's security apparatus has allowed aid to be used as a weapon of war.
In the besieged rebel-held town of Madaya, dozens of civilians starved to death last year after months without any aid deliveries. Although convoys have reached the area since then, doctors say their contents are often ill-suited to needs.
Food deliveries have sometimes carried carbohydrates but no protein, leading to malnutrition among the area's residents.
Government soldiers have also removed antibiotics, anaesthetics and burn treatment kits from trucks bound for areas where operating theatres had run dry, according to UN officials.
On Monday, a coalition of 81 relief groups, most of them Syrian, called for an end to the sieges, with full and unhindered humanitarian access and passage for civilians.
Husam AlKatlaby, a representative of the Violations Documentation Centre, a Syrian human rights group, said the Syrian army had cut off aid to an estimated 450,000 people in the Damascus suburb of eastern Ghouta, an area that attracted global attention in 2013 after a deadly chemical weapons attack by the Syrian Government.
"Patients are already dying due to lack of medicine, food is becoming more expensive, and every day brings fresh casualties from conventional or chemical weapons.
We fear that eastern Ghouta will be the next of many more Aleppos yet to come - cut off, strangled, and bombarded while the world watches," said AlKatlaby, referring to the city retaken by Syrian forces at the end of last year after a punishing months-long siege and bombing campaign.
In a report released on Tuesday, the UN's Commission of Inquiry on Syria accused Assad's army of launching a fresh round of chemical attacks on eastern Ghouta, this time involving chlorine.
Yesterday marked the sixth anniversary of the Syrian revolt, which began as a peaceful anti-government uprising before turning into one of the deadliest armed conflicts of the 21st century. Almost a half-million people have been killed in the intervening years, while much of the country has emptied out.
